The fighting comes nearly three months after the al-Nusra Front expelled another group of Western-backed opposition fighters from Idlib province.
The Syrian Observatory for Human Rights said the militants launched their offensive against the Western-armed Hazem movement on Thursday in Aleppo province.
“The jihadists expelled the rebels from Regiment 111, once a regime army base that Hazem had taken over,” Observatory director Rami Abdel Rahman told AFP.
“Then on Friday, the fighting spread to the north of Idlib province.”
The Hazem group issued a statement confirming the attack.
“The so-called al-Nusra Front attacked the movement’s positions and checkpoints in the west of Aleppo province… We in the Hazem Movement will defend ourselves until the last drop of blood,” it said.
Hazem is mainly present in northern Syria. Last year, it was the first to receive U.S.-made anti-tank missiles from its Western backers.
In November, al-Nusra militants expelled the Western-backed Syrian Revolutionaries Front from Idlib.
Syria’s war began as a popular revolt seeking democratic change, but later morphed into a conflict after President Bashar al-Assad’s regime launched a brutal crackdown against dissent.
A devastating conflict has since broken out. More than 200,000 people have been killed, and half the population have fled their homes.