Twenty-three civilians including nine children were killed in air strikes on a town in eastern Syria held by the Islamic State jihadist group on Thursday, a monitor said.

The Syrian Observatory for Human Rights said it did not know whether Syrian regime or Russian warplanes carried out the strikes on Al-Mayadin in the eastern province of Deir Ezzor.

A US-Russian brokered truce in force since Monday does not include areas where IS is present.