Syria’s fragile ceasefire started to unravel on Sunday with the first aerial attacks on rebel-held neighbourhoods 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 strike on Syrian government 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 Syrian soldiers and led to a harsh verbal attack on Washington by Damascus and Moscow. The US military says it may have unintentionally struck Syrian troops while carrying out a raid against the Islamic State of Iraq and the Levant (ISIL) in eastern Syria on Saturday.

The seven-day ceasefire is supposed to end at midnight Sunday, according to a Syrian army statement issued last week. The US and Russia have said if it holds for seven days, it should be followed by the establishment of a centre for both countries to coordinate the identification of targets against ISIL and Jabhat Fatah al-Sham, formerly known as al-Nusra Front.

The ceasefire has been repeatedly violated by both sides, and aid convoys have not reached besieged rebel-held neighbourhoods of Aleppo, Syria’s largest city and one-time commercial centre.

Moscow laid the blame for Sunday’s violence squarely on the opposition.

Russian Defence Ministry spokesman Major-General Igor Konashenkov said in an emailed statement to the AP news agency that both “terrorists and the opposition” are using the truce to “boost their forces and prepare for renewed hostilities”.

Konashenkov said Moscow still has not been able to contact the US-backed opposition to coordinate ceasefire efforts, despite Washington’s assurances. He said the US has not even tried to get the opposition to hold its fire.

Syria called Saturday’s US-led strikes on the outskirts of the eastern city of Deir Az Zor a “dangerous and blatant aggression against the Syrian Arab Republic and its army”.

The foreign ministry’s statement, sent to the UN Security Council, said American warplanes repeatedly attacked Syrian army positions. It said the air strikes were “on purpose and planned in advance”, and killed dozens of Syrian soldiers.

Russia’s military said it was told by the Syrian army that at least 62 Syrian soldiers were killed in the air raid and more than 100 wounded.

The Russian air force has been carrying out strikes across Syria to bolster President Bashar al-Assad’s forces for nearly a year, and the two militaries work in close coordination.

Russia said the United States was being obstructive and deceptive regarding the air strike. A foreign ministry statement on Sunday said in an emergency UN Security Council session the US took “an unconstructive and indistinct position”.

The Americans “not only turned out to be unable to give an adequate explanation of what happened, but also tried, as is their custom, to turn everything upside down”, the statement said.