Beirut (AFP) - 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.