Washington(CNN) ISIS has claimed responsibility for a deadly explosion that killed four Americans and at least 10 other people in the Syrian city of Manbij on Wednesday.
The American deaths included two US service members, a defense contractor and a Department of Defense civilian, the US Central Command said in a statement. Three other US service members were injured in the attack.
"Initial reports indicate an explosion caused the casualties, and the incident is under investigation," the statement said.
The American civilian was an intelligence expert who was with the troops hoping to collect information about security and adversaries in the area, according to three US officials.
Eight civilians and two fighters from the US-backed Syrian Democratic Forces were killed in the blast, a senior commander from the Manbij military council told CNN.
Prior to Wednesday's attack, only two US service members had been killed in action in Syria since the start of the campaign in 2014.
The US-led coalition confirmed earlier Wednesday that US service members were killed in the attack, but did not provide details regarding the number of casualties at the time and said the service members were "conducting a routine patrol" at the time of the explosion.
"U.S. service members were killed during an explosion while conducting a routine patrol in Syria today. We are still gathering information and will share additional details at a later time," a tweet from Operation Inherent Resolve's spokesperson said.
The names of the two US service members will be withheld for 24 hours until next of kin is notified in accordance with DOD policy, according to CENTCOM's statement.
The ISIS-affiliated Amaq agency said the attack in the northern city of Manbij was carried out by a suicide bomber with an explosive vest.
"An explosion in Manbij's busy market street, initial reports of casualties," spokesman of the Manbij military council Shervan Darwish wrote on Twitter.
ISIS didn't provide any proof it was responsible for the attack.
White House spokeswoman Sarah Sanders said President Donald Trump has been briefed on the situation and issued a statement on the attack later Wednesday.
"Our deepest sympathies and love go out to the families of the brave American heroes who were killed today in Syria. We also pray for the soldiers who were wounded in the attack. Our service members and their families have all sacrificed so much for our country," it said.
Vice President Mike Pence has also been briefed, according to a tweet from his press secretary.
However, Pence made no mention of the attack or the deaths of US service members while making remarks at the Global Chiefs of Mission conference at the US State Department Wednesday, claiming "The caliphate has crumbled, and ISIS has been defeated."
A White House official said the administration had not publicly confirmed the deaths of US service members when Pence made his remarks even though the coalition against ISIS tweeted a message confirming there had been US deaths nearly an hour before his speech.
Pence released his own statement after making those remarks saying, "We will never allow the remnants of ISIS to reestablish their evil and murderous caliphate -- not now, not ever. "
"President Trump and I condemn the terrorist attack in Syria that claimed American lives and our hearts are with the loved ones of the fallen. We honor their memory and we will never forget their service and sacrifice," he said.
However, he also indicated that the White House still intends to withdraw US troops from Syria.
"Thanks to the courage of our Armed Forces, we have crushed the ISIS caliphate and devastated its capabilities. As we begin to bring our troops home, the American people can be assured, for the sake of our soldiers, their families, and our nation, we will never allow the remnants of ISIS to reestablish their evil and murderous caliphate -- not now, not ever," the statement added.
The attack comes less than a month after Trump announced that US troops would withdraw from Syria. In making his announcement, Trump declared in a video released on Twitter: "We have won against ISIS. We've beaten them and we've beaten them badly. We've taken back the land and now it's time for our troops to come back home."
The US has about 2,000 troops in Syria, with no specific date for their withdrawal. Last week, the US began withdrawing some military ground equipment from Syria, according to an administration official with direct knowledge of the operation.
Following Wednesday's attack, two US officials told CNN that there are no current plans to reverse Trump's decision to begin withdrawing US troops from Syria.
The President continues to believe that it's time for US troops to return home, the officials said. Specific withdrawal plans remain contingent on events on the ground, including the strength of ISIS but also security guarantees for the Kurds.
Trump met with with several Republican Senators including Rand Paul, Marco Rubio and Joni Ernst on Wednesday. The meeting was scheduled before Wednesday's attack.
Republican Sen. Lindsey Graham, who was a harsh critic of Trump's plans to bring home US troops from Syria when the decision was announced earlier in December, said Wednesday that he is concerned that the President's statements about withdrawing from Syria have emboldened the enemy.
"My concern, by the statements made by President Trump, is that you set in motion enthusiasm by the enemy we're fighting. You make people we're trying to help wonder about us. And as they get bolder, the people we're trying to help are going to get more uncertain. I saw this in Iraq. And I'm now seeing it in Syria," Graham said during impromptu remarks at a Judiciary Committee hearing on Attorney General nominee William Barr.
"Every American wants our troops to come home, but I think all of us want to make sure that when they do come home, we're safe," he added. "So I would hope the President would look long at hard at where he's headed in Syria. I know people are frustrated, but we're never going to be safe here unless we're willing to help people over there who will stand up against this radical ideology."
During his surprise visit to Iraq on December 26, Trump was warned by military commanders that -- despite his claims -- ISIS was not entirely defeated in Syria. People familiar with the President's reaction said the conversation was eye-opening for a leader who, days earlier, claimed the terror group was defeated "badly" in the country.
The discussion occurred inside a tan tent at the al-Asad airbase west of Baghdad and included the US Ambassador to Iraq Douglas Silliman, Lieutenant General Paul LaCamera, national security adviser John Bolton, and the first lady Melania Trump, along with other officials.
Trump was told that pockets of ISIS militants remained in the Euphrates River valley and that the US military had not yet eliminated all of their strongholds. Commanders told him the US had been successful in taking back other areas but that the job was not finished.
The people familiar with the conversation described it as sobering, and said it broke through to Trump in a way his conversations with national security officials in Washington had not. Coming days after Trump's decision to withdraw troops from Syria, it centered partly on the remaining challenges of going after ISIS fighters in pockets of Syria.
In addition to the briefing from the commanders, Trump found the massive security apparatus on the trip surprising -- something his advisers told him was reflective of the remaining challenges against ISIS.
Still, it remains unclear whether Wednesday's attack will impact Trump's decision to pull US forces from Syria as top administration officials continue to qualify the terms and timing of a pullout -- altering the President's December 19 assertion that forces would leave "now."
After Trump declared that the US would pull troops from Syria and a US Defense official told CNN that planning was underway for a "full" and "rapid" withdrawal, national security adviser John Bolton began adding conditions that could indefinitely delay a troop departure and has refused to discuss timelines.
Secretary of State Mike Pompeo has also avoided offering a specific timeline but insisted in a Cairo speech last week that under the Trump administration, the US is a stalwart presence in the Mideast. He told reporters separately that "there's no contradiction whatsoever" in the US policy on Syria, arguing that "this is a story made up by the media."
At the same time, the top US diplomat has denied allies were confused about the US withdrawal from Syria. "I think everyone understands what the United States is doing," Pompeo said. "At least the senior leaders in their governments do."
Yet on the ground and in diplomatic circles, Trump's decision landed with explosive effect.
US allies in the region were blindsided. Two diplomatic sources say their countries were not consulted or informed and the news came as a total surprise.
But discussions about a US withdrawal have continued this week.
Trump and Turkish leader Recep Tayyip Erdoğan discussed "ongoing cooperation in Syria as US forces begin to withdraw" during a phone call Monday, just one day after Trump threatened to "devastate Turkey economically" if the NATO-allied country attacks Kurds in the region.