Dozens of people were killed earlier this week in a suspected US-led coalition air raid that hit a school sheltering displaced people near Raqqa, according to a monitoring group.

The UK-based Syrian Observatory for Human Rights said on Wednesday that its contacts had counted at least 33 bodies at the site near the village of al-Mansoura, west of Raqqa.

The group, which monitors Syria's war via a network of contacts on the ground, said it believed the air raid at the school-turned-shelter had been carried out by the US-led coalition.

The air raid early on Tuesday morning was also reported by the activist-run Raqqa is Being Slaughtered Silently group, which said dozens of civilians were dead or still missing after the air raid.

A Pentagon spokesman said on Wednesday that the US-led coalition would ask its "civilian casualty team" to investigate the allegations.

The US-led coalition has intensified its aerial campaign against ISIL around Raqqa this month, with numerous civilian casualties reported.

Earlier this month, the coalition said its raids in Syria and Iraq had unintentionally killed at least 220 civilians since 2014. But critics say the number is far higher.

Al Jazeera's Hashem Ahelbarra, reporting from Beirut, said the school in al-Mansoura housed many refugees who had fled the fighting in Raqqa.

The Syrian Democratic Forces (SDF), a Kurdish-led group of militias backed by the US-led coalition, is fighting to isolate Raqqa ahead of an anticipated assault on the city.

Hundreds of thousands of people have been killed and millions more displaced since Syria's conflict erupted in March 2011 with protests against Bashar al-Assad.

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

PHOTO CAPTION

Smoke rises from what activists said were airstrikes by forces loyal to Bashar al-Assad on locations in Hasaka city, Syria July 9, 2015. REUTERS

Al-Jazeera