Syria's cease-fire appeared on the brink of collapse on September 19 after a U.S.-led coalition air strike killed at least 62 Syrian government soldiers during the weekend and the city of Aleppo was hit by its first air raids in nearly a week.
Correspondents say the barrage of strikes on rebel-held districts of Aleppo on September 18 risks reigniting battles in the city 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 truce deal that took effect on September 12.
The co-sponsors of the truce deal -- Russia and the United States -- have each blamed one another.
Relations were strained further after a U.S.-led raid involving Australian planes killed scores of Syrian soldiers on September 17.
Australian Defense Minister Marise Payne said on September 19 that Australia is taking part in a U.S. review of the September 17 air strikes.