Islamic State militants in Syria are said to have abducted 400 civilians during a bloody assault on the eastern city of Deir al-Zour.

The Syrian Observatory for Human Rights, an opposition monitoring body, said people were taken from areas in the north-west of the city.

The report from the London-based group could not be independently verified.

At least 135 people were killed in the attack on Deir al-Zour, the Observatory reported earlier.

It said 85 of them were civilians and 50 were pro-government fighters.

Syrian state media put the toll even higher, saying about 300 civilians had died.

IS launched its attack with suicide bombers, followed by a ground assault.

Syrian government forces retaliated with air strikes and heavy artillery.

The Russian air force has been dropping humanitarian aid for civilians trapped in the city as well as reportedly carrying out air strikes in the vicinity.

The UN recently warned of "sharply deteriorating" conditions in parts of Deir al-Zour, one of a number of towns under siege as a result of Syria's five-year civil war.