Fighters from Al-Qaida's Syrian affiliate and allied groups advanced Friday on a key government-held military base in northwestern Idlib province, a monitor said.
The advance follows the capture of provincial capital Idlib by Al-Nusra Front and its allies on Saturday.
"Violent clashes have been underway since Thursday night between the army and Al-Nusra and its allies... around the Mastuma base," the Britain-based Syrian Observatory for Human Rights said.
The base, seven kilometers (four miles) south of Idlib city, is the biggest regime base in the province, according to Observatory director Rami Abdul Rahman.
The opposition groups had launched a "preventative attack on the base" where regime reinforcements were arriving ahead of a possible bid to recapture Idlib city, he said.