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.

There was no immediate comment from the coalition, but it has said there were 19 strikes near Raqqa on Monday, including three that destroyed IS “headquarters”, and that there were 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”.

Syrian government and Russian aircraft also carry out strikes in Raqqa province.

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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.

Raqqa is Being Slaughtered Silently reported that the school was completely levelled by the strike, and that the 50 families who were sheltering there were still unaccounted for on Wednesday morning.

The anti-IS group also reported that 20 civilians had been killed by coalition air strikes on the town of Tabqa, about 40km (25 miles) west of Raqqa, 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.

Earlier this month, the coalition said its air strikes in Syria and Iraq had unintentionally killed at least 220 civilians since August 2014. However, human rights groups believe the true figure is likely far higher.

Airwars, an organisation that tracks allegations of civilian deaths, reports that at least 2,590 civilians are likely to have died.

Later on Wednesday, new US Secretary of State Rex Tillerson will host the first meeting of the entire 68-member coalition since December 2014.

US officials said the meeting aimed to accelerate efforts to defeat IS in Raqqa and Mosul, the group’s last major urban stronghold in neighbouring Iraq.

Elsewhere in Syria on Wednesday, rebels and allied jihadists continued two surprise offensives inside the capital, Damascus, and north of the city of Hama.