The Syrian Observatory for Human Rights said an air strike early on Wednesday killed at least 30 civilians and injured dozens more in a village held by Islamic State group in eastern Syria.
The strike, in al-Dablan, about 20 km (13 miles) southeast of al-Mayadeen on the west bank of the Euphrates, is the second in 48 hours that the Observatory says has killed dozens of people.
The identity of the jets that carried out the air strike was not known, the Observatory, a Britain-based war monitor, said.
Both a U.S.-led coalition against Islamic State group, and the Syrian military backed by Russia, have targeted the jihadist group in cities and towns along the Euphrates valley.
On Monday a coalition airstrike in al-Mayadeen hit a building used by Islamic State group as a prison, killing 57 people, the Observatory said on Tuesday.
The coalition did not immediately respond to a request for comment on whether its jets carried out Wednesday's strike in al-Dablan. On Tuesday it said it had hit targets in al-Mayadeen the previous day in a mission "meticulously planned" to avoid harming civilians.
It says it takes great pains to avoid harming or killing civilians and investigates all reports that it has done so. The Syrian government and Russia also deny targeting civilians.
The coalition is supporting an offensive by Kurdish and Arab militias against Islamic State group's besieged Syrian capital of Raqqa, 200 km (150 miles) northwest of al-Dablan up the Euphrates.
Syria's army and its allies are pushing through the desert to relieve their own besieged Euphrates enclave in Deir al-Zor, 65 km (40 miles) northeast of al-Dablan. U.S. intelligence officials have said Islamic State group has relocated its leadership to al-Mayadeen.