The bus was ambushed on the road near the village of Sbaikhan in the eastern countryside of Deir al Zour, said the Syrian Observatory for Human Rights.

The Britain-based watchdog group said it's not yet clear who ambushed the bus, but noted that Islamic State (IS) militants are active in that area.

It said the IS militants launched an intensive mortar attack on positions of the Syrian army and pro-government fighters in the desert of Deir al Zour overnight, which triggered the army to fire back.

The IS militants have been actively carrying out attacks against Syrian soldiers and pro-government troops in the desert in the remote eastern countryside of Deir al Zour and the desert of Homs.

A total of 309 Syrian and pro-government soldiers were killed by similar attacks in the desert between March and December of 2019, according to the observatory.

The IS militants have largely been defeated in their key bastions in northern and northeastern Syria.

However, thousands of the IS militants are now hiding in the sprawling desert region in the east of Homs and Deir al Zour.