Al-Nusra captured the town of Khan al-Subul overnight, which was previously controlled by the Hazm Movement - a militia allied to the FSA. Hazm appears to have withdrawn from the town.
Al-Nusra also seized another five villages held by FSA rebel groups over the weekend, according to the Observatory, a London-based opposition group.
The latest Nusra advance comes after it took control of several strategic towns and villages in the Jabal al-Zawiya area of Idlib from the Syrian Revolutionaries Front (SRF), another FSA-aligned group, on Saturday.
Among the towns captured was Deir Sinbul, which was home to the SRF headquarters.
Blow to Syrian opposition
The SRF is led by Jamal Maarouf, a well-known figure among the Syrian opposition, and a man deeply reviled by al-Nusra, as well as the Islamic State group (IS, formerly known as ISIS).
In a video released on Saturday, Maarouf said that his rebels
You [al-Nusra] killed more people than the regime killed. - Jamal Maarouf, SRF chief |
"We withdrew from Deir Sinbul to preserve the blood of civilians.
"You [al-Nusra] killed more people than the regime killed," Maarouf said.
The Observatory, which relies on a wide network of Syrian sources for its information, said that tens of SRF fighters had defected and joined al-Nusra.
Asad Kanjo, an Idlib-based activist, said that the Nusra advance was "a blow to the moderate Syrian opposition in general".
Both Hazm and the SRF are backed by the West, with Hazm reportedly even receiving supplies of US-made weapons, including TOW anti-tank missiles.