BEIRUT: Syria's cease-fire was on the brink of collapsing Sunday, after a U.S.-led coalition strike killed dozens of government 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 cease-fire 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 cease-fire's co-sponsors, Russia and the United States, have traded accusations over its fraying, with relations strained even further after the U.S.-led raid killed scores of Syrian soldiers on Saturday.
Russia said the situation in Aleppo city 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.
The Syrian Observatory for Human Rights said four strikes on Aleppo city killed one woman and left several people wounded, but it could not identify who carried them out.
An AFP correspondent in Karam al-Jabal neighbourhood saw several wounded children after a raid.
Also on Sunday, a senior adviser to President Bashar al-Assad said Damascus believes Saturday's strike that killed the Syrian soldiers 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 al-Zor. Moscow put the death toll at 62.
The Syrian army said the raid had allowed ISIS fighters to gain ground around a key eastern airbase, but a military source said government forces were back on the offensive Sunday.
"The army has retaken most of its positions on Jabal Therdeh with Russian and Syrian air support," the source said, referring to the hilltop.
Recapturing the positions is vital to stop ISIS using them to fire on warplanes taking off or landing at the airbase.
On Sunday, ISIS said it shot down a Syrian warplane near Deir al-Zor.
State media confirmed a plane had been shot down and its pilot killed, but did not say who was responsible.
The base and adjacent government-held neighbourhoods of Deir al-Zor city have been under siege since 2012 and have been dependent on resupply by air.
- 'Bad omen' for cease-fire -
Hours after the coalition strike, the Pentagon admitted U.S.-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."
"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 (ISIS) terrorists," it said.
It again called on Washington to force rebels to sign on to the truce, warning that "otherwise, the realization of Russia-U.S. agreements ... could be put in danger."
An emergency U.N. Security Council meeting called by Moscow to discuss the attack ended early on Saturday after an exchange between the U.S. and Russia reminiscent of Cold War-era verbal jousting.
U.S. ambassador Samantha Power said Moscow's request for the meeting was a "stunt," while her Russian counterpart Vitaly Churkin accused the U.S. of violating agreements that it would not target army positions.
Churkin called the strike a "bad omen" for the U.S.-Russia deal on halting the war in Syria, which has killed more than 300,000 people since it erupted in 2011.
"These strikes endanger everything that has been done so far by the international community" to end the war, said Russian foreign ministry spokeswoman Maria Zakharova.
- Fighting on multiple fronts -
The Observatory had reported a drop in violence across all major fronts where ISIS was not present.
But fighting flared again late last week, including in the central provinces of Homs and Hama, and the rebel-held eastern suburbs of Damascus.
Nine people including a child were killed Sunday when a pair of barrel bombs hit the opposition-held town of Daael in the southern province of Daraa, the Observatory said.
The head of Jabhat Fatah al-Sham, which changed its name from the Nusra Front after renouncing its ties to 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-government groups to band together in a single organization.
Such a merger would throw a major wrench in the U.S.-Russia deal, which foresaw cooperation between the two world powers against extremists, including Fatah al-Sham and ISIS, if the truce holds for a week.