Beirut: Syria`s ceasefire appeared on the brink of collapse on Monday after a US-led coalition strike killed dozens of regime soldiers and Aleppo city was hit by its first air raids in nearly a week.
The barrage of strikes on rebel-held districts of Aleppo on Sunday risks reigniting battlefronts in the city and could be the most serious threat to the ceasefire so far.
A halt to fighting around Aleppo and the delivery of desperately needed humanitarian aid were key components of the fragile deal that took effect last Monday evening.
The ceasefire`s co-sponsors, Russia and the United States, have each blamed one another, with relations strained even further after the US-led raid killed scores of Syrian soldiers on Saturday.
Syria accused the United States of sabotaging the ceasefire.
"The objective of this US aggression is to bring about the failure of the truce," said Bashar Jaafari, Syria`s ambassador to the United Nations.
Sunday was the deadliest day of the truce so far, said the Syrian Observatory for Human Rights, with 11 civilians killed in areas where the ceasefire was supposed to have taken hold.
Russia said Aleppo was "especially tense", blaming the instability on rebels.
"The amount of shelling by rebel groups against positions of Syrian government troops and of residential areas is increasing," said defence ministry spokesman Igor Konashenkov.
French Foreign Minister Jean-Marc Ayrault accused Syrian President Bashar al-Assad`s government of undermining the ceasefire.The evening strikes on Aleppo killed one woman and wounded others, said the Britain-based Observatory, which could not identify who carried them out.
An AFP correspondent in Aleppo`s Karam al-Jabal district saw several wounded children after a raid.
Ten people including a child were killed Sunday when a pair of barrel bombs hit an opposition-held town in the southern province of Daraa, the Observatory said.
"Today was the highest death toll since the truce began," said Observatory director Rami Abdel Rahman.
The deaths took the number of civilians killed across Syria on Sunday to 11, and up to 26 since the ceasefire began on Monday, he added.
Tensions between the US and Russia escalated after the raid by the American-led coalition killed dozens of Syrian soldiers.
A senior adviser to Assad said that Damascus believed the strike was "intentional".
"None of the facts on the ground show that what happened was a mistake or a coincidence," Buthaina Shaaban told AFP.
The Observatory said at least 90 soldiers were killed in the strike on a strategic hill near Deir Ezzor. Moscow put the death toll at 62.
The Syrian army has been fighting off an Islamic State group offensive around the key Deir Ezzor airbase since last year.
On Sunday, IS said it shot down a Syrian warplane near the city.
State media confirmed a plane had been shot down and its pilot killed, but did not say who was responsible.Hours after the coalition strike, the Pentagon admitted US-led pilots may have hit Assad`s forces but said that they "believed they were striking a Daesh (IS) fighting position".
Russia said it was "deeply concerned", warning that Washington would have to rein in rebels fighting Assad "otherwise, the realisation of Russia-US agreements... could be put in danger".
"The actions of the pilots -- if they, as we hope, were not taken on orders from Washington -- fall between criminal negligence and direct pandering to IS terrorists," it said.
An emergency UN Security Council meeting called by Moscow to discuss the attack saw an exchange between the US and Russia reminiscent of Cold War-era verbal jousting.
The situation is likely to loom large at the annual UN General Assembly starting Tuesday as well as a UN summit on migration on Monday.
US ambassador Samantha Power said Moscow`s request for Sunday`s emergency meeting was a "stunt", while her Russian counterpart Vitaly Churkin accused the US of violating agreements that it would not target army positions.
Churkin called the strike a "bad omen" for the US-Russia deal to halt Syria`s war, which has killed more than 300,000 people since it erupted in 2011.
Amid the spike in tension, food aid reached the Damascus suburb of Moadamiyat al-Sham after a government deal granting amnesty to opposition fighters in the besieged town.
Moadamiyat al-Sham mayor Bassam Karbuj said about 7,000 food parcels were distributed and that the army would take full control of the town once remaining rebels are bussed out "in the coming days".