The conservative Islamic factions, including the al-Qaida-affiliated Nusra Front, are coordinating a multi-pronged campaign whose main target appears to be the town of Jisr al-Shughour in Idlib province.
Opposition fighters are also attacking government checkpoints in a sprawling agricultural plain south of the town as well as nearby military facilities.
The operation keeps the pressure on beleaguered government forces in the area just weeks after the rebels captured Idlib city, the provincial capital.
The opposition groups taking part in the new operation posted a statement online late Wednesday announcing the start of the offensive, which they call "Battle of Victory." They appealed to residents to stay inside "until God grants us and you victory and liberation."
Also Read
The Britain-based Syrian Observatory for Human Rights said Thursday the rebels have taken five checkpoints since the offensive began, including two near Jisr al-Shugour, in fighting that has killed at least eight rebels and an unknown number of government troops.
A Twitter account tied to the Nusra Front in the area carried videos showing what it said were fighters firing heavy machine guns at government forces during the clashes. It also posted photos of tanks and artillery purportedly shelling Syrian military positions.
The material could not be immediately verified, but it appeared genuine and corresponded to other Associated Press reporting.
Syria's state news agency said Thursday that the army prevented "terrorist groups" from sneaking into military camps in Idlib, and killed "scores of terrorists" in the countryside around Jisr al-Shughour. The government refers to those fighting to topple Assad as terrorists.