BEIRUT (Reuters) - Russian warplanes bombed Syrian rebel-held areas in northwestern Syria and government forces pounded a suburb of the capital on Thursday, ahead of a planned halt to fighting that rebels predicted Damascus and Moscow would ignore.

Still image taken from aerial footage released by Russia's Defence Ministry on February 11, 2016, shows airstrikes, carried out by the country's air force and hitting what the Defence Ministry says was Islamic State militants' stronghold, in Latakia, Syria. REUTERS/Ministry of Defence of the Russian Federation/Handout via Reuters

The “cessation of hostilities” agreed to by the United States and Russia is due to take hold on Saturday morning from midnight. But opponents of President Bashar al-Assad say they expect the government to press on with its advance, by branding opposition fighters al Qaeda militants unprotected by the truce.

Damascus has agreed to the deal, as has the main opposition alliance, although it is only ready to commit for two weeks given its deep reservations. But the government and its allies will be permitted to forge on with strikes against jihadist militants of Islamic State and an al Qaeda-linked group, the Nusra Front.

The government also says the agreement could fail if foreign states supply rebels with weapons or insurgents use the truce to rearm.

U.S. President Barack Obama said on Thursday the United States was resolved to try to make the deal work but that “there are plenty of reasons for scepticism.”

Saying the Syrian government and Russia must live up to their commitments, Obama told reporters after meeting with his national security team: “The coming days will be critical and the world will be watching.”

Ending the conflict in Syria, he added, could allow all parties to focus on the fight against Islamic State.

Fighting in the final days before the truce has focused on Daraya, a besieged suburb of the capital held by fighters the government describes as Nusra militants but rebels say are from other groups, and on the northwest near the Turkish frontier.

Four months of Russian air strikes turned momentum Assad’s way in a 5-year-old war that has killed more than 250,000 people, created the world’s worst refugee crisis and seen Islamic State fighters declare a “caliphate” in Syria and Iraq.

The multi-sided civil war has drawn in most regional and global powers, with Western countries, Arab states and Turkey forming a coalition against Islamic State while also backing rebels fighting to overthrow Assad. Russia and Iran support him.

BARREL BOMBS

The Syrian Observatory for Human Rights, a British-based group that monitors the conflict, said army helicopters dropped at least 30 “barrel bombs” on Daraya on Thursday. Assad’s opponents say the army drops oil drums filled with explosives and shrapnel to cause indiscriminate harm in rebel areas.

The government blamed groups linked to Nusra for firing mortars into residential areas of Damascus, killing at least one person.

A spokesman for rebels in southern Syria predicted Daraya would be the first place where the truce would collapse.

“They want to exploit the ceasefire and focus their fire on Daraya to take it. This will be the first breach. We won’t accept it,” said Abu Ghiath al-Shami, spokesman for the Alwiyat Seif al-Sham group, part of a rebel alliance in the south.

A Syrian military source also signaled that Damascus would not stop fighting in Daraya.

“There is evidence that the ones there are Nusra Front. They found documents, books, flags that point to the Nusra Front being in Daraya,” the military source said. “In any place where there is Nusra Front, we will continue operations.”

Fighting has also escalated in the past two days in the northwestern province of Latakia, where Free Syrian Army groups backed by Assad’s foreign enemies operate close to Nusra fighters and other jihadists.

“The regime wants to try to retake all of northern Latakia before Feb. 26,” said Fadi Ahmad, spokesman for the First Coastal Division rebel group, speaking to Reuters from the area.

‘VERY FIERCE’ BATTLES

“The battles are very fierce. Yesterday, there were heavy battles in the part of rural Latakia that is still with us,” he said, adding he did not expect the government or its Russian allies to abide by the truce: “Three minutes ago, I saw a Russian plane in the sky hitting us here in rural Latakia.”

The Syrian military source also said operations were taking place in the northern Latakia area.

Recapturing areas of Latakia province at the Turkish border has been a top priority for Damascus and its allies since Russia began its strikes. It is one of several areas where the government has made major gains this year.

Rami Abdulrahman, director of the Observatory, confirmed heavy air strikes in northern Latakia on Wednesday and Thursday.

He predicted the presence of the Nusra Front and like-minded groups would give the government grounds to press on with fighting there under the agreement.

One of the main purposes of the cessation of hostilities is to allow aid to reach civilians, especially in besieged areas cut off from supplies.

A U.N. airdrop of food to 200,000 people in the besieged city of Deir al-Zor failed on Wednesday, with all 21 palettes dropped by parachute either damaged, landing in no-man’s land or unaccounted for, a U.N. World Food Programme spokeswoman said.

U.N. adviser Jan Egeland nevertheless said the cessation of hostilities could rescue the civilian population from “the abyss” and end the “black chapter” of sieges.

Assad told Russian President Vladimir Putin on Wednesday his government was ready to help implement the halt to fighting. The two leaders nevertheless stressed the importance of an “uncompromising” fight against Islamic State, the Nusra Front and other jihadists not party to the truce.

‘NO PLAN B’

Russian officials have seized on comments by U.S. Secretary of State John Kerry that Washington would consider a “Plan B” if the ceasefire failed. Russia’s RIA news agency quoted Foreign Minister Sergei Lavrov as saying there was no such “Plan B”.

Foreign Ministry spokeswoman Maria Zakharova accused “some U.S. officials” of trying to “sabotage” the ceasefire plan.

But after more than five years of failure to negotiate any end to fighting, and with Russia’s intervention having had a decisive impact on the ground, it was not clear what sort of fallback plan Washington might consider if the truce fails.

Bob Corker, chairman of the U.S. Senate Foreign Relations Committee and a Republican critic of the Obama administration, said of Putin: “I think he understands there’s no ‘plan B’.”

The Russians were “now dominating,” Corker told MSNBC. “It’s totally in Russia’s hands now.”

U.N. Syria envoy Staffan de Mistura said he would announce on Friday a date for a new round of talks between Syria’s warring parties. The last talks were called off this month before they got under way, with rebels saying they could not talk while government troops advanced and Russia bombed.