Syria's nearly week-old ceasefire was facing a crucial deadline on Monday as the Syrian army said the truce could expire later in the day after a particularly bloody weekend.
The deal came into effect on the evening of September 12, backed by world powers hoping that it could bring an end to Syria's brutal five-year conflict.
But after several days of relative calm, fighting escalated across major battlefronts, culminating in a deadly US-led air raid on a Syrian army position and fresh strikes on Aleppo.
The strike on Syrian soldiers has strained relations between the deal's primary sponsors, Washington and Moscow, who had said they would increase cooperation against jihadist groups if the truce held for a week.
A senior military source in Damascus told AFP that the truce would expire Monday at 7:00 pm (1600 GMT) if no extension was announced.
"The current situation leaves much to be desired," Kremlin spokesman Dmitri Peskov said, blasting the US-led strike on Syria's army which he said "is still the only party respecting the ceasefire".
"All of this puts the ceasefire in danger," he said, also without commenting on whether the truce would be extended.
Since September 12, a total of 26 civilians, including eight children, have been killed in areas where the truce had been set to take hold, according to the Syrian Observatory for Human Rights.
The bloodiest day for civilians was Sunday, when a barrel bomb attack killed 10 in a southern rebel-held town and one woman was killed in the first raids on Aleppo since the truce began.
On Monday, fierce clashes were reported from all across Damascus as Syria's military launched "a major military operation" against rebels.
"The army blocked a huge attack by the Faylaq al-Sham group and is beginning a major military operation on the edges of the capital," a military source said.
But forces loyal to President Bashar al-Assad were once again on the back foot near the eastern city of Deir Ezzor, where they have been battling Islamic State group jihadists since last year.
The Observatory said IS militants seized Jabal Therdeh, a set of hills overlooking the Deir Ezzor airbase -- from Russia-backed Syrian soldiers on Sunday.
A military source in Deir Ezzor told AFP on Monday the army had begun a new operation backed by Russian and Syrian warplanes to roll back IS's advance.
"The army has redeployed around Jabal Therdeh after a fierce attack by Daesh," he said, using the Arabic acronym for IS.
Syria's military accused the US-led air coalition of paving the way for IS, after a strike by coalition forces on Jabal Therdeh killed at least 62 Syrian soldiers on Saturday.
Senior government adviser Buthaina Shaaban told AFP on Sunday Damascus believed the raid had been "intentional".
Russia called the strike a "bad omen" for its deal with the US to halt Syria's war, which has killed more than 300,000 people since it erupted in 2011.
Under the agreement, fighting was to halt across Syria and humanitarian aid would reach civilians living in increasingly dire humanitarian conditions.
On Monday, convoys of food and medical aid were en route to two hard-to-reach areas in Syria, said David Swanson, a spokesman for the UN Office for the Coordination of Humanitarian Affairs.
The first would reach the rebel-held town of Talbisseh, where at least two people were killed by shelling during the truce.
Swanson said the aid would be enough for 84,000 people in the area.
Another 78,000 people living in and around Greater Orum in the north of Aleppo province would also receive wheat flour and health supplies, Swanson told AFP.
But aid convoys to the rebel-held districts of Aleppo city were still stuck on Syria's border with Turkey Monday, he said.
UN humanitarian chief Stephen O'Brien said he was "pained" that Aleppo's residents had still not received promised aid deliveries.
"I am pained and disappointed that a United Nations convoy has yet to cross into Syria from Turkey, and safely reach eastern Aleppo, where up to 275,000 people remain trapped without food, water, proper shelter or medical care," O'Brien said.