Syria's ceasefire was on the brink of collapsing Sunday, after a US-led coalition strike killed dozens of regime soldiers and Aleppo city was hit by its first raids in nearly a week.
The barrage of strikes on rebel-held districts of Aleppo risks reigniting battlefronts there 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 on Monday evening.
The ceasefire's co-sponsors, Russia and the United States, have traded accusations over its fraying, with relations strained even further after the US-led raid killed scores of Syrian soldiers on Saturday.
Russia said the situation in Aleppo was "especially tense" on Sunday, blaming the instability on rebels.
"The amount of shelling by rebel groups against positions of the Syrian government troops and of residential areas is increasing," said defence ministry spokesman Igor Konashenkov.
But French Foreign Minister Jean-Marc Ayrault accused Bashar al-Assad's government of undermining the ceasefire.
"We must not forget that it is first of all the (Syrian) regime, and it is always the regime, which has jeopardised the US-Russian ceasefire," he said in New York.
Sunday was the deadliest day of the truce so far, according to the Syrian Observatory for Human Rights, with 10 civilians killed in areas where the ceasefire was set to take hold.
'Intentional' US-led strike?
The evening strikes on Aleppo killed one woman and wounded others, said the Britain-based monitor, 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.
Nine 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. Ten were killed today out of 25 civilians killed in total since Monday," said Observatory head Rami Abdel Rahman.
Tensions between the US and Russia were escalating Sunday after a raid by the American-led coalition killed dozens of Syrian soldiers the previous evening.
A senior adviser to Assad said on Sunday that Damascus believes 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 (IS) 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.
'Bad omen' for ceasefire
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".
It said coalition forces "would not intentionally strike a known Syrian military unit."
Russia said it was "deeply concerned," warning that Washington would have to reign 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 ended early on Saturday after an exchange between the US and Russia reminiscent of Cold War-era verbal jousting.
US ambassador Samantha Power said Moscow's request for the 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.
Meanwhile, head of Fateh al-Sham Front, which changed its name from Al-Nusra Front after breaking ties with Al-Qaeda, said late Saturday that "neither we nor rebel groups will allow the siege of Aleppo to continue."
Abu Mohamed al-Jolani told Al-Jazeera negotiations were under way for anti-regime groups to band together in a single organisation.
Such a merger would throw a major wrench in the US-Russia deal, which foresaw cooperation between the two world powers against jihadists, including Fateh al-Sham and IS, if the truce holds for a week.
