Those killed late Thursday in the village of Sousa in the eastern province of Deir Ezzor were mostly foreigners, said the Syrian Observatory for Human Rights.

The Britain-based monitor, which relies on sources inside Syria for its information, was unable to say if the strikes were carried out by a US-led coalition fighting IS or Iraq.

The Iraqi army said it had targeted an IS "operations room" inside Syria late Thursday, killing "a number of IS elements", but did not mention a precise location.

It was not immediately clear whether the Iraqi army and the Observatory were referring to the same incident.

IS overran large swathes of Syria and neighbouring Iraq in 2014, proclaiming a "caliphate" in land it controlled.

But the jihadist group has since lost most of it to various offensives in both countries. In Syria, its presence has been reduced to pockets in Deir Ezzor and parts of the vast Badia desert.