Syrian troops have recaptured parts of a military base seized by rebels early this year in fierce fighting near Aleppo international airport, a monitoring group said on Friday.
Heavy fighting erupted at 4 (0200 GMT) around Base 80 outside Syria's main northern city, the Syrian Observatory for Human Rights said.
The troops were backed by militia and fighters from Lebanese militant group Hezbollah in their assault on rebel groups including Iraq and the Levant, the Observatory said.
"Regime troops have advanced inside the base and now control large parts of it, and rebel groups and the State are bringing in reinforcements," it said.
Initial reports suggested both sides had suffered casualties while towns near the base had come under heavy bombardment.
Rebel forces seized control of Base 80 in heavy fighting in February.
The base's garrison was responsible for securing Aleppo international airport and the adjacent Nayrab military airfield, which both remain under regime control.
More than 120,000 people have been killed in the conflict in Syria since March 2011, according to the Observatory. Millions more have been internally displaced or fled abroad.