(CNN) As much of the world's gaze is focused on the battle for Mosul, Kurdish military groups and their allies aligned with the US announced an operation to liberate the terror group's de facto Syrian capital of Raqqa.
The Syrian Democratic Force (SDF) said in a statement they had established a joint operations center Saturday night for the military campaign "Euphrates Rage," an attempt by the US-backed coalition to strike at the heart of ISIS.
Coalition spokesman Col. John Dorrian told CNN that rebel forces allied to the US will begin by working positions east of Raqqa, to shut down the so-called "back door escape" out of both Mosul and Raqqa.
"They have begun their march toward Raqqa," he said.
While Mosul is symbolic as ISIS' last major possession in Iraq -- and the city from which ruler Abu Bakr al Baghdadi announced the creation of his caliphate -- Raqqa holds as much significance for the terror group.
"The importance of Raqqa is that is where ISIS plans their external (terrorist) operations," Gen. Joseph Votel, the commander of US Central Command told CNN national security analyst Peter Bergen. It is the largest Syrian city still under ISIS control -- and first captured by the militant group.
"Raqqa is recognized as the financial, leadership and external ops center of the Islamic State, so that's what makes it important."
Raqqa is home to nearly 200,000 people, most Sunni Arabs, and an estimated 5,000 militants, according to the group Raqqa is Being Slaughtered Silently (RSS), one of the only sources of information to come out of the city.
Life inside Raqqa
In this photo from November 29, 2015, provided to CNN by the activist group Raqqa is Being Slaughtered Silently, residents assess the damage to a building in the northern Syrian city -- ISIS's headquarters -- which has been the target of French airstrikes in recent weeks.
In this photo from November 6, 2015, an ISIS fighter walks along a street in Raqqa with his 3 wives walking behind him, according to RBSS.
A child looks at a stand selling military fatigues in Raqqa on October 1, 2013. Many in Raqqa say they don't want to live under ISIS but have no choice.
Men look at a large black jihadist flag in Raqqa on September 28, 2013. In Raqqa today, school is banned -- and even small pleasures, like chocolate, are an unaffordable luxury because many cannot work.
A Syrian man mourns the deaths of six of his siblings, killed in a bomb attack during fighting between rebel fighters and Syrian government forces in Raqqa on August 10, 2013.
A man carries two children away from the scene of an explosion in Raqqa on August 7, 2013.
In this undated photo, provided to CNN by RBSS, you can see normal life in Raqqa -- once one of Syria's most liberal cities -- before the start of the civil war.
The streets of Raqqa before it was under ISIS control, in an undated photo provided to CNN by RBSS.
Raqqa siege could be toughest yet in fight against ISIS
Who is taking the fight to ISIS in Raqqa?
The biggest challenge is finding troops to bring the fight on the ground against ISIS. Most analysts believe that Raqqa will present a different set of challenges from the Mosul campaign because of the absence of local troops able to carry out the assault.
No indigenous allied force currently exists near Raqqa, although the SDF, which has consisted primarily of ethnically Kurdish troops -- primarily Kurdish militias with strong ties to the Kurdistan Workers' Party, or PKK -- has expanded its ranks to include a wider coalition of Syrian rebel groups.
The group, which is seen as largely synonymous with the Kurdish YPG, is viewed with suspicion by the Sunni-majority residents of Raqqa, who are concerned that a SDF victory in their city could lead to forced displacements.
"The SDF is the partner force most capable of acting soon to isolate Raqqah and commenced movement toward Raqqah on 5 November," a statement from the Coalition Joint Task Force-Operation Inherent Resolve said, using an alternate spelling for the Syrian city.
"We believe the inclusion of fighters from the local population is an important advantage to the SDF."
The SDF includes multiple Arab, Kurdish and Turkmen militia groups and will be carried out in coordination with coalition forces, including airstrikes.
US Defense Secretary Ash Carter stressed the need for local forces to take the lead, saying, "We want a victory that sticks everywhere, so it's always local forces."
A staunchly secular Kurdish militia in Syria known as the People's Protection Units, or YPG, has proven to be one of ISIS' deadliest enemies on the ground in Syria.
The Kurds' stiff resistance to the ISIS siege of the border town of Kobane in 2014 prompted the US to send help in the form of airstrikes and weapons.
The Kurdish faction has since succeeded in capturing large swaths of territory from ISIS.
At alternating times the Kurdish militia also has periodically clashed with, and cooperated with, Syrian rebel groups and the Syrian government.
In Syria, up to 300 US Special Operations Forces advising the SDF are authorized to be in the country.
The ISIS terror threat
People flee the scene of a terror attack at Istanbul's Ataturk airport on June 29. Turkish officials have strong evidence that ISIS leadership was involved in the planning of the attack, a senior government source told CNN. Officials believe the men -- identified by state media as being from Russia, Uzbekistan and Kyrgyzstan -- entered Turkey from the ISIS stronghold of Raqqa in Syria, bringing with them the suicide vests and bombs used in the attack,
the source said.
The ISIS militant group -- led by Abu Bakr al-Baghdadi, pictured -- began as a splinter group of al Qaeda.
Its aim is to create an Islamic state, or caliphate, across Iraq and Syria. It is implementing Sharia law, rooted in eighth-century Islam, to establish a society that mirrors the region's ancient past. It is known for killing dozens of people at a time and carrying out public executions.
Kurdish Peshmerga fighters fire missiles during clashes with ISIS in Jalawla, Iraq, on June 14, 2014. That month, ISIS took control of Mosul and Tikrit, two major cities in northern Iraq.
Traffic from Mosul lines up at a checkpoint in Kalak, Iraq, on June 14, 2014. Thousands of people
fled Mosul after it was overrun by ISIS.
ISIS fighters parade down an Iraqi street in this image released by the group in July 2014.
Aziza Hamid, a 15-year-old Iraqi girl, cries for her father while she and other Yazidi people are flown to safety after a
dramatic rescue operation at Iraq's Mount Sinjar on August 11, 2014. A CNN crew
was on the flight, which took diapers, milk, water and food to the site where as many as 70,000 people were trapped by ISIS. Only a few of them were able to fly back on the helicopter with the Iraqi Air Force and Kurdish Peshmerga fighters.
On August 19, 2014, American journalist James Foley
was decapitated by ISIS militants in a video posted on YouTube. A month later, they released videos showing the executions of American journalist Steven Sotloff and British aid worker David Haines.
ISIS militants stand near the site of an airstrike near the Turkey-Syria border on October 23, 2014. The United States and several Arab nations
began bombing ISIS targets in Syria to take out the group's ability to command, train and resupply its fighters.
A Kurdish marksman stands atop a building as he looks at the destroyed Syrian town of Kobani on January 30, 2015. After four months of fighting, Peshmerga forces
liberated the city from the grip of ISIS.
Safi al-Kasasbeh, right, receives condolences from tribal leaders at his home village near Karak, Jordan, on February 4, 2015. Al-Kasasbeh's son,
Jordanian pilot Moath al-Kasasbeh, was burned alive in a video that was released by ISIS militants. Jordan is one of a handful of Middle Eastern nations taking part in the U.S.-led military coalition against ISIS.
In February 2015, British newspapers report the identity of "Jihadi John," the disguised man with a British accent who had appeared in ISIS videos executing Western hostages. The militant was identified as Mohammed Emwazi, a Kuwaiti-born Londoner. On November 12, 2015, the Pentagon announced that Emwazi was in a vehicle
hit by a drone strike. ISIS later confirmed his death.
In March 2015, ISIS released video and images of a man being thrown off a rooftop in Raqqa, Syria. In the last photograph, the man is seen face down, surrounded by a small crowd of men carrying weapons and rocks. The caption reads "stoned to death." The victim was brutally killed
because he was accused of being gay.
An Iraqi soldier searches for ISIS fighters in Tikrit on March 30, 2015. Iraqi forces
retook the city after it had been in ISIS control since June 2014.
Dead bodies lie near a beachside hotel in Sousse, Tunisia, after
a gunman opened fire on June 26, 2015. ISIS claimed responsibility for the attack, which killed at least 38 people and wounded at least 36 others, many of them Western tourists. Two U.S. officials said they believed the attack might have been inspired by ISIS but not directed by it.
ISIS also claimed responsibility for what it called a suicide bombing
at the Al-Sadiq mosque in Kuwait City on June 26, 2015. At least 27 people were killed and at least 227 were wounded, state media reported at the time. The bombing came on the same day as the attack on the Tunisian beach.
A man inspects the aftermath of a car bombing in Khan Bani Saad, Iraq, on July 18, 2015.
A suicide bomber with an ice truck, promising cheap relief from the scorching summer heat, lured more than 100 people to their deaths. ISIS claimed responsibility on Twitter.
Two women hold hands after an explosion in Suruc, Turkey, on July 20, 2015. The blast
occurred at the Amara Cultural Park, where a group was calling for help to rebuild the Syrian city of Kobani, CNN Turk reported. At least 32 people were killed and at least 100 were wounded in the bombing. Turkish authorities said they believed ISIS was involved in the explosion.
Spectators at the Stade de France in Paris run onto the soccer field after explosions were heard outside the stadium on November 13, 2015. Three teams of gun-wielding ISIS militants
hit six locations around the city, killing at least 129 people and wounding hundreds.
Law enforcement officers search a residential area in San Bernardino, California, after a
mass shooting killed at least 14 people and injured 21 on December 2, 2015.
The shooters -- Syed Rizwan Farook and his wife, Tashfeen Malik -- were fatally shot in a gunbattle with police hours after the initial incident. The couple supported ISIS and had been planning the attack for some time, investigators said.
Two wounded women sit in the airport in Brussels, Belgium, after two explosions rocked the facility on March 22, 2016. A subway station in the city
was also targeted in terrorist attacks that killed at least 30 people and injured hundreds more. Investigators say the suspects belonged to the same ISIS network that was behind the Paris terror attacks in November.
A boy walks past bloodstains and debris at a cafe in Balad, Iraq, that was attacked by ISIS gunmen on May 13, 2016. Twenty people were killed.
Iraqi government forces patrol in southern Falluja, Iraq, on June 10, 2016. In late June,
a senior Iraqi general announced that the battle to reclaim Falluja from ISIS had been won.
What US Special Forces are doing on the ground in Syria and Iraq
How will the fight for Raqqa complement Mosul operations?
The two cities -- ISIS' last remaining Iraqi stronghold and its proclaimed capital in northern Syria -- are around 360 km (224 miles) apart. The supply line between the two is vital to the jihadists' continued defense of the territory it holds.
Already, Shia militia involved in the offensive around Mosul have cut access to the main road leading from Mosul to western Iraq and toward Syria.
"It will be some time before they reach the city. In the meantime we will continue shaping operations like airstrikes against Da'esh leaders, command and control and resources. This is the first step in a campaign that will be conducted deliberately," Votel added, using another term for ISIS.
"Doing operations simultaneously with the campaign to liberate Mosul will complicate command and control for the enemy, giving them more problems to solve than their flagging command and control can manage," Votel said.
While the city of Raqqa is smaller than Mosul, the battle to retake it is expected to be fierce. It's ISIS' heartland and also sits in the middle of the country's ongoing civil war, which includes foreign actors such as the US and Russia.
The campaign will proceed in "very deliberate phases," the first of which will be an "isolation phase," according to Special Presidential Envoy for the Global Coalition to Counter ISIL Brett McGurk.
Coalition planes will provide air support while ground operations are to be undertaken by local militia under the SDF banner.
US 'laying groundwork' for Raqqa assault before Mosul capture
Who's out?
Presumably, the Syrian government -- which claims to want to reunite the fractured country under its authority -- would lead a proposed offensive against a nihilistic movement like ISIS.
However, the government of Bashar al-Assad has been at war with his own people for five straight years.
Throughout this conflict, the military has suffered untold casualties and large-scale desertions.
The secretary of defense also ruled out Russia's participation in the fight for Raqqa, saying that Moscow "is not a participant in our Raqqa plan."
The Russian-Syrian government alliance has appeared far more focused on battling other Syrian rebel groups across the country. These allies have been laying siege for two months to the rebel-controlled part of the Syrian city of Aleppo.
US, Russia spar amid charges of war crimes in Syria
What is Turkey's role?
The rise of the Kurds has also become a complicating factor. Many in Turkey see the establishment of Kurdish "statelets" in northern Syria as a major threat.
The YPG have close links to the Kurdistan Workers' Party, or PKK, Kurdish separatist rebels who have waged a guerrilla war against the state in Turkey for thirty years.
Turkey has declared itself an enemy of ISIS. Turkish warplanes periodically bomb ISIS targets in Syria, while also arresting suspected ISIS operatives in Turkey.
The SDF alliance also includes the YPG, which Turkey considers a terrorist group.
However, Turkey also routinely attacks the YPG in Syria, arguing that the Kurdish militia is as much a threat to Turkish national security as ISIS.
The Turkish-backed Free Syrian Army (FSA) have, in addition to making gains against ISIS, been clashing against Kurdish groups -- including the SDF and YPG.
Opinion: Why is Turkey detaining Kurds?
CNN's Peter Bergen, Ryan Browne and Angela Dewan contributed to this report.