Syrian activists say the Islamic State group has launched an offensive against government forces in an eastern city and captured several buildings.

The Local Coordination Committees, an activist-run collective, says IS fighters have captured the Assad hospital, university dorms and grain silos in their offensive on Saturday in the contested city of Deir el-Zour.

The Britain-based Syrian Observatory for Human Rights says the fighting killed at least 20 government troops and six IS fighters. It says government forces have surrounded the hospital but the fate of its staff and patients is not known.

Deir el-Zour, near the border with Iraq, is split between government forces and IS fighters.