DAMASCUS - At least 33 people have been killed in a United States-led airstrike on a school in northern Syria, said a monitor group on Wednesday.

The school, which was hit on Tuesday, was being used to house displaced people in the town of Mansura near Raqqa, the Islamic State group's de facto capital, said the Syrian Observatory for Human Rights.

The UK-based watchdog said activists claimed the bodies of 33 victims were pulled from the rubble on Wednesday.

The group said that the coalition had recently increased airstrikes on IS-held areas in Raqqa.

The US coalition previously admitted there had been civilian casualties due to the strikes against IS-held areas.

Top officials from the 68-nation alliance fighting IS are set to meet in Washington on Wednesday to hear more about US President Donald Trump's plan to destroy the jihadists' remaining strongholds in Iraq and Syria.

The US-led coalition has been bombing IS since 2014 and is now backing a major offensive to defeat the group in Raqqa.

The school-turned-shelter hit on Tuesday morning lies about 30 kilometers west of Raqqa.

"They're still pulling bodies out of the rubble until now. Only two people were pulled out alive," said Observatory head Rami Abdel Rahman.

The British-based monitor - which relies on a network of sources inside Syria for its information - says it determines what planes carried out raids according to their type, location, flight patterns and the munitions involved.

"Raqqa is being slaughtered silently," an activist group that publishes news from IS-held territory in Syria said of the strike.

"The school that was targeted hosts nearly 50 displaced families," it said.

Syrian state news agency SANA also reported the air raid, accusing the US-led coalition of inflicting "dozens" of casualties and almost completely destroying the school site.

Earlier this month, the coalition said its raids in Syria and Iraq and unintentionally killed at least 220 civilians.

But other monitors say the number is far higher.

Trump has ordered his top generals to craft an accelerated strategy to "eradicate" IS's so-called caliphate, and allied countries are keen to learn more at Wednesday's meeting.

Coalition partners are expected to provide feedback on a revised anti-IS plan drafted by the Pentagon and presented to Trump last month.