Pro-government forces in Syria clashed with Islamic State militants around a military air base in the north of the country on Tuesday as they tried to break the jihadists' siege on the facility, a group monitoring the war said.

The Syrian Observatory for Human Rights said forces fighting on the side of the Syrian government, including Lebanese and Iranian militiamen, were trying to secure Kweires air base, which has been under attack for nearly two years.

There were some initial reports that the pro-government forces had managed to enter inside the walls of the base after heavy clashes, the Observatory said.