Al-Qaeda's Syrian affiliate Al-Nusra Front launched an offensive Friday on the headquarters of a new US-formed rebel group in the country's north, leaving at least 11 dead, a monitor said.

The attack came one day after the Al-Nusra kidnapped eight rebels from Division 30, who had been trained and equipped by Washington, said the Syrian Observatory for Human Rights monitoring group.

"Al-Nusra attacked at dawn the headquarters of Division 30, near the town of Azaz in the northern province of Aleppo," the Britain-based Observatory said.

Clashes broke out after the assault killing five Al-Nusra fighters and allied militants as well as six rebels who were defending Division 30, said Observatory director Rami Abdel Rahman.

He said Al-Nusra launched the attack to seize weapons "given by Washington to the rebels" but that  it was unable to enter the base as clashes were still underway.

At least 54 members of Division 30 entered Syria in mid-July, equipped with 30 American-made all-terrain vehicles, arms, and ammunition.

Eight of them, including a commander, were kidnapped on Thursday by Al-Nusra in a village in Aleppo province, the Observatory said.

On its Facebook page, the Division denounced the kidnapping of Colonel Nadim Hassan and his companions, demanding "the brothers of Al-Nusra Front to liberate them immediately".
A spokeswoman for the US Department of Defence denied the kidnapping.

"I can confirm that there have been no New Syrian Force personnel captured or detained," said Commander Elisa Smith.

The "New Syrian Force" is the term used by Washington to describe Syrian rebels who have been screened to exclude extremist elements and passed a training course led by US troops.

More than 230,000 people have been killed in Syria since the conflict began in March 2011 with anti-government protests that descended into a civil war after a regime crackdown.
Source: AFP