The US-led coalition denied it had launched the strike after Syrian government claims
At least 22 people were killed late on Monday in strikes by unidentified aircraft on a Syrian government-held neighbourhood in the eastern city of Deir Ezzor, a monitor said.
A local official, and Syrian state television, accused the US-led coalition fighting the Islamic State (IS) group of carrying out the strikes, and gave a lower death toll of 14, with 30 injured.
IS holds part of the city, but the deaths came in a neighbourhood under government control and appeared to be the possible result of a mistake, said the Britain-based Syrian Observatory for Human Rights.
The war monitor said the 22 civilians were killed in the al-Qusur neighbourhood, in the west of the city, which is provincial capital of oil-rich Deir Ezzor province.
A government source in the city, speaking to AFP on condition of anonymity, accused the US-led coalition of the air strikes.
"Fourteen civilians were killed and more than 30 others injured in air strikes by coalition planes on the al-Qusur neighbourhood in Deir Ezzor city," the source said, adding that the toll could rise because a number of the injured were in serious condition.
But the US-led coalition denied it was responsible for the air strike.
"The allegation that a coalition strike may have killed 14 civilians and wounded 32 others in Deir Ezzor is false," coalition spokesman Ryan Dillon told AFP.
"Russian-backed pro-regime forces are conducting operations in Deir Ezzor and the coalition does not support pro-regime operations," he said.
Dillon said the coalition had carried out only one strike in the area in the last two months, on 16 September in support of US-backed forces fighting IS east of the city.
Syrian state television, citing its reporter, also accused the coalition of the strikes.
Syrian government forces backed by ally Russia have seized most of Deir Ezzor city after breaking an IS siege of nearly three years on government-held districts in September.
The government offensive against IS, backed by Russian air power, is being waged largely on the western side of the Euphrates river that cuts diagonally across Deir Ezzor province.
A second, separate offensive against the militants is being fought on the eastern side by the Kurdish-Arab Syrian Democratic Forces, backed by the US-led coalition against IS.
A "deconfliction" mechanism is meant to keep the two campaigns separate.
IS controls less than half of Deir Ezzor province, its last remaining stronghold in the country after the SDF ousted it from its bastion Raqqa last week with US-led coalition support.
More than 330,000 people have been killed in Syria since the war began in March 2011 with anti-government protests.