Islamic State has launched its fiercest assault in a year against a besieged Syrian government enclave in the city of Deir al-Zor, trying to cut it off from a nearby military air base in a battle that has killed dozens.

A Syrian military source said the fighting was intense. "This attack is one of the most fierce attempts by Daesh on the airport and the area," the source told Reuters, using an Arabic term for the group. "It seems Daesh has mobilized a big force."

Describing the fighting as the fiercest in the eastern city for a year, the Syrian Observatory for Human Rights said on Monday that at least 82 people had been killed.