A woman walks through rubble after airstrikes in the rebel-held Syrian town of Dael on Friday. (Alaa Al-Faqir/Reuters)

Questions about how and when a Syrian truce will be implemented deepened skepticism that the cease-fire will work, as Russian and Syrian warplanes on Friday sustained their relentless bombardment of rebel-held territory in the country.

Syrian opposition groups cautiously welcomed the deal reached by world powers at a meeting in Munich on Thursday but said they were concerned that it allows Russia to continue its air campaign against the Islamic State and Jabhat al-Nusra terrorist groups — and perhaps also against moderate rebels.

The deal marks the first attempt to bring about any kind of pause to the fighting since a U.N.-backed cease-fire in 2012 collapsed within hours, and world leaders expressed hope that this agreement would herald the beginning of an end to the nightmarish war.

But many key details remain unaddressed, including when exactly the truce will begin, who will enforce it and whether the factions on the ground will accept it.

“It is an agreement full of holes and ambiguities,” said Emile Hokayem of the London-based International Institute for Strategic Studies. “This is an agreement struck because of an emergency, but it lacks any real political dimension.”

Syrians were not a party to the agreement, which was hammered out by Russia, the United States, and key European and Middle Eastern powers. Among them were regional rivals Saudi Arabia and Iran, two of the chief sponsors of the factions on the ground, lending hope that this effort may succeed where others have failed to bring an end to the bloodshed.

The deal calls for a temporary cessation of hostilities to begin in a week, the delivery of humanitarian aid to besieged communities on both sides of the conflict and the resumption of stalled peace talks in Geneva later in the month.

But the language on the timing was vague, and by late Friday, there was still no firm indication when any of the deal’s provisions would go into effect.

“What we have here are words on paper,” Secretary of John Kerry cautioned after the deal was announced. “What we need to see in the next few days are actions on the ground.”

Syrian rebel groups said they would meet in the coming days to decide whether to accept the deal. There was no immediate response from the Syrian government, which has said in the past that it will not agree to any kind of cease-fire until the “terrorists” fighting to topple President Bashar al-Assad are defeated.

In an interview with the French news agency Agence France-Presse conducted hours before the agreement was reached, Assad was defiant, saying the goal of his government was to reconquer all of Syria from the “terrorists” who have seized many parts of the country.

The United States, Russia and other powers came to an agreement on a ‘cessation of hostilities’ in Syria, but the deal was met with caution and skepticism. (Jason Aldag,Ishaan Tharoor/The Washington Post)

Assad’s confidence has revived since Russia’s intervention in September reversed the fortunes of his wearied, depleted army. The airstrikes have enabled government loyalists to make significant advances­ against the rebels in several key areas, most recently in the vicinity of the province of Aleppo.

Those advances continued Friday with Russian warplanes striking multiple locations across Syria, including in the northern countryside of Aleppo in support of a 10-day-old offensive aimed at laying siege to the rebel-held portion of the city of Aleppo.

Residents of the areas of northern Aleppo that have borne the brunt of the bombing campaign expressed dismay that the cease-fire would not come into effect for a week.

“Within a week everything will have been destroyed,” said Mohammed Najjar, a resident of the town of Marae. On Friday, he joined an accelerating exodus of tens of thousands of civilians toward the Turkish border, where they have been blocked by Turkish authorities from entering the country.

Much of the skepticism hinged on long-standing differences between the United States and Russia over which groups in Syria’s war count as “terrorist.” U.S. officials say Russia has directed about 70 percent of its airstrikes against moderate rebel groups, some of them backed by the United States. But Russia claims that all have been aimed at terrorists.

“The Russians target moderate groups and say we are Daesh,” said Lt. Col. Ahmed Saoud, the leader of the U.S.-backed Division 13 rebel group, using the Arabic acronym for the Islamic State. He cited as an example an attack on one of his group’s bases­ in a Russian bombing in the northwestern province of Idlib in October, which Russian military officials announced as a strike on an Islamic State base.

“I hope it will succeed,” he said of the agreement. “I wish for anything that will stop the bloodshed. We want a cease-fire and a political solution — but not just any cease-fire. We need international monitoring.”

The recent escalation of Russia’s air campaign has only deepened rebel suspicions of Russia’s intentions, said Issam Rayess, a spokesman for the U.S.-backed Southern Front, which has also lost ground to Russian-backed offensives near Syria’s border with Jordan.

“Even now they continue to indiscriminately bomb civilians and our moderate groups into the dust, and they say they are hitting terrorists,” he said. “We no longer trust words.”

After the government advances­ in recent weeks, the rebels may, however, have no choice but to accept the deal, Saoud said.

“We are the weakest part of this chain, and we don’t have any cards,” he said. “If the Americans and the Russians agreed, then we will obey. We can only listen to their orders.”

The U.N. special envoy for Syria, Staffan de Mistura, said humanitarian agencies hoped to implement the first phase of the agreement, calling for humanitarian deliveries of aid to besieged communities, where people have been starving to death, “within the coming days.”

The text of the agreement reached Thursday called for the aid deliveries to begin “this week,” which implied they were supposed to begin Saturday.

Karen DeYoung in Brussels and Brian Murphy in Washington contributed to this report.

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