The Syrian Observatory for Human Rights said Al-Nusra fighters captured the town of Khan al-Subul after the withdrawal of the Hazm movement, a moderate opposition group.
The Al-Qaeda affiliate also seized another five villages in the province held by Islamist and moderate rebel groups, the Observatory said.
The advance comes a day after the Britain-based Observatory reported Al-Nusra fighters had seized the Idlib bastion of the Syria Revolutionaries Front (SRF), another Western-backed opposition group.
The Observatory said Saturday that Al-Nusra had captured Deir Sinbel and most of the other towns and villages in the Jabal al-Zawiya area of Idlib province.
The advance of the Al-Qaeda affiliate is seen as a setback to US efforts to create and train a moderate rebel force as a counterweight to jihadists and the forces of Syrian President Bashar al-Assad.
The Observatory said some members of the SRF even deserted the group during the fighting and joined Al-Nusra's ranks.
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The Hazm movement is among the rebel groups that have received limited supplies of US-made weapons, including TOW anti-tank missiles.
The Observatory said Al-Nusra had seized weapons during its advance in Idlib province, but it was not immediately clear if US-made weapons were among those captured.
Syrian moderate and Islamist rebels have in the past allied with Al-Nusra in battles against both the Syrian regime and the jihadist Islamic State group.
But since July, sporadic clashes have occurred between the Al-Qaeda affiliate and other rebel groups, particularly in Idlib province.
The fighting began shortly after Al-Nusra's chief announced the group's intention to establish an Islamic "emirate," intended to rival the "caliphate" established by the Islamic State group.
Al-Nusra and rebels have also fought in the southern region of Daraa and Aleppo province in the north.
Despite a fierce rivalry between Al-Nusra and the Islamic State group, the Observatory said individual IS jihadists had fought alongside Al-Nusra in the recent clashes against the SRF in Idlib.