The Syrian Observatory for Human Rights, a British-based monitoring group, said on Thursday that at least 38 civilians were killed in air strikes carried out by a US-led coalition in Hasaka province in northeast Syria in the past two days.
The US and its allies are carrying out air raids in the area against Islamic State, which controls some parts of Hasaka province but has lost ground in recent months to a US-backed alliance that includes the powerful Kurdish YPG militia.
Hasaka neighbours mostly Islamic State-held Deir al-Zor province and Raqqa, the group's de-facto capital in Syria.
The death toll published by the Observatory, which tracks the war using a network of contacts on the ground, included at least 15 people killed when strikes hit a bakery in the city of al-Shadadi near the border with Iraq on Tuesday.
Air raids in at least three other villages killed 15 others on Thursday, including three children, while eight more civilians died in air strikes elsewhere, it said. Reuters could not independently confirm the reports.
Separate raids near the town of al Houl close to the Iraqi border and farther south killed 35 Islamic State fighters, the Observatory said.
The US-led coalition has also been hitting Islamic State areas of control in neighbouring Iraq.
Washington backs the Syria Democratic Forces in Syria, an alliance of Kurdish YPG fighters and other groups fighting the jihadists on the ground, and which took al Houl from Islamic State in November.
The YPG has been the most effective partner against IS in Syria for the US-led coalition, and took swathes of territory from the group last year.
The SDF announced on its Facebook page on Thursday it was launching another offensive, this time to capture al-Shadadi, a logistics hub for the Islamic State group, which is located on a network of highways. Its capture would isolate Raqqa, the de-facto capital of the hardline group.
Russia is separately carrying out its own air campaign in Syria, hitting some Islamic State targets, but mostly focused on insurgents fighting Moscow's ally President Bashar al-Assad in the west of the war-torn country.