The Syrian branch of al-Qaeda have advanced near the Syria-Turkey border, in an apparent attempt to seize a vital crossing under control of western-backed rebels.

The Nusra Front edged to less than 6km from the strategic Bab al-Hawa crossing on Monday, a key supply line to Syrian rebels fighting in the northern Idlib province.

The UK-based Syrian Observatory for Human Rights, an activist group which monitors the country's civil war, said fighters from the Nusra Front were seen gathering in the nearby town of Sarmada.

Held by the moderate rebel alliance known as the Islamic Front, Bab al-Hawa is an important supply route in delivering food and medical supplies to residents of northern Syria.

The potential move by the Nusra Front deals another blow to US plans of building a moderate rebel force capable of fighting both President Bashar al-Assad's forces and the Islamic State of Iraq and the Levant (ISIL).

The advance comes a day after the Nusra Front overran a town and several villages in Idlib province, seizing them from the Syria Revolutionaries Front (SRF).

The recent defeats were a sharp blow for the western-backed SRF, headed by prominent commander Jamal Maarouf, and Harakat Hazm.

Harakat Hazm, a group armed and funded by the US, emerged earlier this year with online videos showing its fighters using western-donated weapons, including US anti-tank weapons. It has made modest advances in Idlib.

Battle for Kobane

The Nusra Front's bitter rival, ISIL, has seized a third of both Iraq and Syria, and has been focused on trying to take the eastern Syria border town of Kobane from Kurdish fighters.

About 150 Iraqi-Kurdish Peshmerga entered the embattled town on Thursday armed with heavy weaponry, including anti-aircraft machine guns, and mobile rocket launchers.

Al Jazeera's Jamal Elshayyal, reporting from the Turkey-Syria border, said local Kurdish fighters, known as the YPG, were being supported by the Peshmerga in opening a new frontier on the southwestern side of the town.

"Peshmerga fighters are using mortars as the YPG are advancing on foot," Elshayyal said.

"But with less than 200 fighters aiding the effort to fight ISIL, it's difficult to see what they [Peshmerga] can add to a battle that has seen US air strikes and drones ineffective against ISIL."

On Monday, four air raids targeted ISIL positions near Kobane, striking a small number of fighters, the group's fighting positions, and buildings controlled by the group, according to US Central Command.

ISIL's offensive on Kobane and nearby villages has killed more than 800 people, activists say, and forced more than 200,000 people to flee to neighbouring Turkey.

The Syrian civil war, which began as a peaceful uprising against Assad's rule more than three years ago, has become a bloody and protracted sectarian conflict killing more than 200,000 people.

Source: Al Jazeera and agencies