Washington(CNN) Two days after world powers agreed to a "cessation of hostilities" in Syria, prospects for its success are already being met with skepticism as bombs pelted Damascus suburbs Friday and officials involved acknowledged the difficulties ahead.
Finer points of the deal are being worked out in Geneva by U.S.- and Russian-led task forces that aim to rush humanitarian aid to besieged areas and force a pause in the 5-year-old civil war within a week.
Spurred by the spiraling humanitarian disaster that has left more than 250,000 dead and 13.5 million Syrians in need of help, per U.N. calculations, the 20-member International Syrian Support Group set a goal in Munich, Germany, on Thursday to expedite aid deliveries with an emphasis on hard-to-reach areas, possibly beginning as soon as this weekend.
But the plan to stop the fighting faces a slew of challenges, not least of them Syrian President Bashar al-Assad, who told Agence France-Presse on Friday that he intends to retake "the whole country" from rebel forces.
Diplomats and analysts are concerned about what could happen in the days before the ceasefire takes effect and expressed doubts that hundreds of smaller militias -- or even the Russians -- will actually stop fighting.
If they do pause, it could simply be to rearm and reposition before renewing the onslaught that has displaced millions, destabilized the Middle East and unsettled Europe.
"We're very skeptical" the cessation of hostilities will work, said one European diplomat, who spoke anonymously to discuss sensitive matters.
"We've seen before with Russia that they tend to not necessarily respect this kind of agreement," he said.
Secretary of State John Kerry, an architect of the agreement, acknowledged Thursday night that it's "ambitious." He explained that the International Syria Support Group had scaled back its goals, deliberately avoiding a formal ceasefire that has tougher, legally binding requirements.
The test, he said, is what happens on the ground in the coming days.
"What we have here are words on paper," he told reporters after the agreement was reached. "What we need to see in the next few days are actions on the ground in the field."
His predecessor -- and current Democratic presidential candidate -- Hillary Clinton said she is skeptical that Russia is acting in good faith and that the violence will halt.
"Let's hope that we can accelerate the ceasefire, because I fear that the Russians will continue their bombing, try to do everything they can to destroy what's left of the opposition," she said during a Democratic presidential debate on Thursday in Milwaukee, Wisconsin.
Some experts were even more pessimistic.
"You can see all the pitfalls in this," said Joshua Landis, a Syria expert and director of the Center for Middle East Studies at the University of Oklahoma. "It beggars the imagination how it's going to happen."
Russian support for its longstanding ally Assad has stabilized his struggling regime to the point where it is now taking back territory and has almost encircled rebels in the northern city of Aleppo. U.S. officials and diplomats privately fear Russia will use the next week to seize more territory.
Andrew Tabler, a senior fellow at the Washington Institute for Near East Policy, said fighting could intensify as Russia and Syria try to make strategic use of the days before the ceasefire starts.
"It's unclear what's going to happen on the battlefield before the ceasefire," Tabler said. "They could try to close the ring around Aleppo" or take other steps that weaken the opposition, leaving it without much leverage when the time finally comes for negotiations.
"A lot of those developments could impact humanitarian deliveries and other elements in the agreement," he said.
There is wide skepticism in Europe about the Munich deal and Russia's intentions, according to diplomats and analysts there who have seen Russia enter ceasefire agreements in Ukraine only to help separatists violate them within days.
"Russia's commitment to a ceasefire should not be taken for granted by the other ceasefire agreement signatories," said Alex Kokcharov, a senior Russia analyst at IHS Country Risk, a global consultancy firm.
Kokcharov pointed to a ceasefire agreement in Minsk, Ukraine, a year ago that was undermined within six days when "pro-Russian separatists, and, as evidence from multiple sources suggests, regular Russian troops," used the pause to take over a strategic rail hub.
"This just comes it a very good time for Russia," the European diplomat noted, referring to gains that Moscow has helped Assad make.
The United States, insulated from the conflict and refugee flows by an ocean and continent, doesn't feel the same urgency about Syria that Europe does, the diplomat said, adding that many Europeans feel the United States hasn't taken tough enough steps to end the violence there.
The Munich agreement feels no different, he said. "It doesn't feel like we're going to get there."
Fighting is expected to continue against Jabhat al Nusra and ISIS, which are excluded from the cessation of hostilities because they are terrorist organizations, Kerry said in Munich. Any other group deemed a terrorist organization by the U.N. Security Council will also not be protected.
Russia and Assad have said they will continue to strike at terrorists even as they observe the pause in fighting and work toward negotiations.
"We have fully believed in negotiations and in political action since the beginning of the crisis; however, if we negotiate, it does not mean that we stop fighting terrorism," Assad told the AFP. "The two tracks are inevitable in Syria: first, through negotiations, and second through fighting terrorism. And the two tracks are separate from each other."
Complicating matters, the United States and Russia have clashed over the definition of a terrorist. Obama administration officials say Russia is striking moderate U.S.-backed groups, a charge Moscow has denied.
That means violence in northern Syria and around Aleppo, where the al Qaeda affiliate al Nusra has a strong presence, will likely continue, according to Columb Strack, another IHS analyst.
If the cessation of hostilities does manage to take hold elsewhere, Landis of the University of Oklahoma said it will likely be eggshell fragile because so many groups are involved in the fighting.
"There are 1,500 militias out there that are all going to have to abide by this ceasefire," he said. "They've had no input into this."
If the violence does ebb, that pause will just be used by all sides to retrench, regroup and prepare for further fighting, Landis said, with rebels trying to get arms in from suppliers in Turkey and Saudi Arabia and the Russians and Iranians doing what they can to bolster the Syrian army.
"Both sides will be trying to take advantage of this pause to strengthen their position," he said.