An air strike on Thursday evening killed at least 35 civilians, including family members of Islamic State fighters, in Al-Mayadin, a town held by the jihadists near Deir al-Zor in Syria, a war monitor reported.
Most air raids against Islamic State strongholds along the Euphrates basin in Syria have been conducted by a U.S.-led coalition and by Syrian and Russian jets.
Iraqi warplanes have also carried out at least one air raid in east Syria.
The deaths bring the total number of people killed in two days of aerial bombardment in Al-Mayadin to 50 said the monitor, the Britain-based Syrian Observatory for Human Rights.
Residents saw reconnaissance aircraft and warplanes circling the city at 7.25 p.m. (1625 GMT) before they fired missiles which struck two buildings, one of which was a four-storey block housing Syrian and Moroccan families of Islamic State fighters.
Islamic State is losing ground in both Syria and Iraq under assault from an array of sometimes rival forces in both countries.
Many of its fighters who have retreated from other fronts are amassing in Syria’s Euphrates basin area.
The U.S.-led coalition against Islamic State says it is careful to avoid civilian casualties in air strikes and investigates any that are reported to have taken place.