Air strikes by the US-led coalition fighting the Islamic State group killed 20 civilians in Syria's eastern Deir Ezzor province, a monitor said on Tuesday.
The Syrian Observatory for Human Rights said the deaths came in two separate incidents on Monday.
It also reported 10 civilians, among them nine children, were killed in a suspected Russian air strike on Tuesday on a town in the rebel-controlled province of Idlib.
The Observatory, which relies on a network of sources inside Syria for its information, says it determines whose planes carry out raids according to type, location, flight patterns and munitions used.
The Britain-based monitor said a US strike on Monday night on the Deir Ezzor town of Albu Kamal had killed 13 civilians, among them five children.
The strike also killed three members of IS, which controls the town by the Syria-Iraq border, the monitor said.
Earlier Monday, a US-led coalition strike killed seven civilians, including a child, in the village of Husseinyeh, the monitor said.
The US-led coalition has been carrying out air strikes against IS in Syria since 2014 and is providing air support for a Kurdish-Arab alliance advancing on the jihadist bastion of Raqa.
Last month, the coalition said its campaign against IS in Syria and Iraq had unintentionally killed at least 220 civilians, but monitors say the real number is far higher.
Most of the oil-rich province of Deir Ezzor, in Syria's east, is held by IS, including parts of the provincial capital, Deir Ezzor city.
The jihadists have besieged the remaining government-held parts of Deir Ezzor city, trapping civilians inside with limited access to supplies.
More than 320,000 people have been killed in Syria since the conflict began with anti-government protests in March 2011.