At least 33 people were killed in a US-led coalition strike on a school used as a centre for displaced people near a jihadist-held Syrian town, a monitor said Wednesday.

The Syrian Observatory for Human Rights said the strike south of Al-Mansoura, a town held by the Islamic State group in the northern province of Raqa, "took place in the early hours of Tuesday."

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 Raqa city, the Syrian heart of the group's so-called "Islamic caliphate".

The school-turned-shelter hit on Tuesday morning lies about 30 kilometres (20 miles) west of Raqa.

"We can now confirm that 33 people were killed, and they were displaced civilians from Raqa, Aleppo and Homs," said Observatory head Rami Abdel Rahman.

"They're still pulling bodies out of the rubble until now. Only two people were pulled out alive," Abdel Rahman told AFP.

The Britain-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.

"Raqa is Being Slaughtered Silently," an activist group that publishes news from IS-held territory in Syria, also reported 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.

More than 320,000 people have been killed and millions more displaced since Syria's conflict erupted in March 2011 with protests against President Bashar al-Assad.

The conflict has since evolved into a multi-front war that has facilitated the rise of jihadist groups and drawn in international powers, including the US.

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.