Reuters/Beirut

 At least 100 soldiers and fighters were killed in northern Syria in just over 24 hours of fierce fighting earlier this week as pro-government forces reached an airbase that had long been besieged by Islamic State, a monitoring group said on Thursday.

Most of those killed were Islamic State militants, followed by government forces and its allies, the Britain-based Syrian Observatory for Human Rights said.

The breaking of the siege was the most high-profile military victory for Syria's government since Russia intervened in the conflict with an air campaign on Sept. 30.

On Tuesday government troops reached the Kweires airbase in the northern province of Aleppo, where soldiers and officers had been holed up for nearly two years.

Recent offensives by government forces mostly against insurgents not linked to Islamic State in the west and northwest of Syria have been backed by Russian air strikes, as well as Iranian troops and Lebanese Hezbollah fighters.

But their progress has been limited at best, with rebels putting up fierce resistance and even advancing in some areas, such as in Hama province where certain areas have come under heavy bombardment.