The clashes, near a Syrian Revolutionary Front bastion in Deir Sinbel village, 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 group.
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 Revolutionary Front have relations with the West and want the 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!"
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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 President Bashar al-Assad's regime in 2011.
Meanwhile, 150 Iraqi peshmerga fighters, who arrived Thursday on the Turkish side of the border, were still waiting to cross and join fellow Kurds defending the town of Kobane.
The Observatory said Syrian Kurdish fighters advanced on an IS post in the north of the town, leaving many dead among the jihadists.
In the centre of town, the IS blew up a car bomb near Kurdish positions, it said.
The Observatory also reported that artillery pieces were destroyed and IS fighters killed as US-led coalition air strikes targeted the north and centre of Kobane.