The Syrian branch of al-Qaeda, al-Nusra Front, has seized a town and several villages in Syria's Idlib province, dealing another blow to western-backed rebel fighters in the northwestern region.

The Syrian Observatory for Human Rights on Sunday said that al-Nusra fighters had captured the town of Khan al-Subul overnight after the withdrawal of the Hazm movement, a moderate opposition group.

The group also seized another five villages in the province held by other rebels, the Observatory said.

The Syrian government holds and clings to the unity and the territorial integrity of Syria. This is for us a sacred issue. - Bouthaina Shaaban, Assad adviser

The advance comes a day after the Observatory reported that al-Nusra fighters had seized the Idlib bastion of the western-backed Syria Revolutionaries Front (SRF).

"We pulled out of Deir Sunbul to spare the blood of civilians," said Jamal Maarouf, commander of the SRF.

Weapons seized

The advance of al-Nusra is seen as a setback to US efforts to create and train a moderate rebel force as a counterweight to more radical groups and President Bashar al-Assad's forces.

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

The Observatory said al-Nusra had seized weapons during its advance in Idlib, but it was not immediately clear whether US-made weapons were among those captured.

The three-year war in Syria has killed an estimated 200,000 people and divided the country. Rebel groups fighting the regime have taken over large swathes of land while the Islamic State of Iraq and the Levant holds territory in the north and east.

Meanwhile, the Syrian regime insisted it was still in control.

"The Syrian government holds and clings to the unity and the territorial integrity of Syria. This is for us a sacred issue," Assad adviser Bouthaina Shaaban said.