Moderate rebels from the Syrian Revolutionary Front and Al-Qaeda affiliate Al-Nusra Front fought in the northwestern province of Idlib Friday, a monitor said, as the groups exchanged bitter insults online.
The clashes, near a Front bastion in the village of Deir Sinbel, came days after Al-Nusra took over Front checkpoints, according to the Syrian Observatory for Human Rights.
"Both sides suffered heavy losses," said the Britain-based monitor.
On Thursday, another clash pitted the moderate Hazzem movement against Al-Nusra in Khan al-Sibel, also in Idlib, killing three fighters.
Both Hazzem and the Syrian Revolutionaries Front have relations with the West, and call for establishment of a democratic state in Syria.
They are also fiercely critical of jihadist groups.
Activists published a video showing the Front's chief, Jamal Maarouf, dressed in military gear and walking among fighters.
He addresses Al-Nusra leader Abu Mohammad al-Jolani, saying "you have tarnished the name of Islam, and you have tarnished religion. Why do you fight us? Go away, fight the regime!"
He added: "You are nothing; you are just like Baghdadi... you bastard."
Maarouf was referring to Abu Bakr al-Baghdadi, who heads the Islamic State (IS) group, known for its horrific atrocities.
Al-Nusra hit back on Twitter, accusing Maarouf of "corruption" and of "straying from the path of the revolution."
Idlib province was among the first areas to fall from government control, soon after the outbreak of a revolt against Assad's regime in 2011.