BEIRUT (AFP) - Al-Qaeda affiliate Al-Nusra Front seized a town in Syria's Idlib province overnight, in another blow to Western-backed rebel fighters in the northwestern region, a monitor said Sunday.

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 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 on 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 blow to US efforts to create and train a moderate rebel force as a counterweight to Islamist militants 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.

The Hazm movement is among the rebel groups that have received limited supplies of US-made weapons, including TOW anti-tank missiles.

Syrian moderate and Islamist rebels have in the past allied with Al-Nusra in battles against both the Syrian regime and the Islamic State in Iraq and Syria (ISIS).

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 ISIS.

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 ISIS, the Observatory said individual ISIS fighters had fought alongside Al-Nusra in the recent clashes against the SRF in Idlib.