Syria's fragile ceasefire continued to unravel on Sunday with the first aerial attacks on opposition-held neighborhoods of Aleppo and a southern village in the deadliest day since the truce began.

The violations came as tensions between American and Russian brokers of the deal worsened following a deadly US air raid on Syrian regime forces.

The UK-based Syrian Observatory for Human Rights said evening air strikes on Aleppo killed one woman and wounded others, though it could not identify who carried them out.

Ten people, including a child, were killed on Sunday when a pair of barrel bombs hit an opposition-held town in the southern province of Deraa, it said.

"Today was the highest death toll since the truce began," said Observatory director Rami Abdel Rahman.

The air raid by the US-led coalition allegedly killed dozens of regime soldiers and led to a harsh verbal attack on Washington by Damascus and Moscow. The US military says it may have unintentionally struck Syrian regime troops while carrying out a raid against ISIL in eastern Syria on Saturday.

The ceasefire has been repeatedly violated, and aid convoys have not reached besieged opposition-held neighborhoods of Aleppo, Syria's largest city and one-time commercial center.

PHOTO CAPTION

A general view shows Castello road in Aleppo, Syria September 16, 2016. REUTERS

Al-Jazeera