At least 33 people have been killed in an air strike on a school in a village west of the Islamic State-held Syrian city of Raqqa, a monitoring group says.
The Syrian Observatory for Human Rights said the building in al-Mansoura was being used as a shelter for displaced people when it was hit on Monday night.
The activist group Raqqa is Being Slaughtered Silently said the fate of 50 families who were there was unknown.
Both groups believe the raid was carried out by US-led coalition jets.
The coalition said it had no indications that an air strike hit civilians but that it would investigate the reports of casualties, the Associated Press reported.
The coalition acknowledged that there were 19 strikes near Raqqa on Monday, including three that destroyed IS "headquarters", and another 18 strikes on Tuesday.
The coalition is supporting an offensive by an alliance of Syrian Kurdish and Arab fighters aimed at capturing Raqqa, the de facto capital of the IS "caliphate".
As part of an operation to isolate the city, US helicopters airlifted Arab members of the Syrian Democratic Forces (SDF) behind IS lines on Wednesday in an attempt to take the Tabqa dam, about 45km (28 miles) to the west, the Pentagon said.
The Syrian Observatory, which is based in the UK, said one of its activists had witnessed 33 bodies being pulled out of the rubble of the al-Badiya school in al-Mansoura, about 26km (16 miles) west of Raqqa.
Two other people were found alive before IS militants arrived and told bystanders to leave, it added.
Residents of the village told the Syrian Observatory that displaced families from Raqqa, Homs and Aleppo provinces were living in the school.
The anti-IS group also reported that 20 civilians had been killed by coalition air strikes on the town of Tabqa on Tuesday.
The Syrian Observatory said the al-Mansoura air strike meant at least 116 civilians, including 18 children and 23 women, had been killed in suspected coalition air strikes in the past two weeks.
Raqqa is Being Slaughtered Silently put the civilian death toll at 101, but did not appear to include any of those feared dead in al-Mansoura.
Airwars, an organisation that tracks allegations of civilian deaths, reports that at least 2,590 civilians are likely to have died.
Syrian government and Russian aircraft also carry out strikes in Raqqa province.
Later on Wednesday, new US Secretary of State Rex Tillerson opened the first meeting of the entire 68-member coalition since December 2014.
Mr Tillerson said the Trump administration was determined to "demolish and destroy" what he called a "barbaric terrorist organisation".
Elsewhere in Syria on Wednesday, rebels and allied jihadists continued two surprise offensives inside the capital, Damascus, and north of the city of Hama.