Al-Qaeda’s Syria affiliate launched a fierce assault on the headquarters of a US-trained rebel group today, while claiming responsibility for capturing members of the "moderate" force earlier this week.

Al-Nusra Front unleashed its offensive on the base of the Division 30 unit in northern Aleppo province, a monitor said, sparking clashes and air raids by a US-led coalition that killed at least 18 of the jihadists.

Seven rebels died defending the base, which Al-Nusra failed to capture, according to the Syrian Observatory for Human Rights.

In a statement on Facebook, Division 30 said five of its members had been killed and 18 wounded in the fight for the base.

It called on rebels to come to its aid in defending the base, and demanded that Al-Nusra stop "spilling the blood of Muslims." 

Al-Nusra accused Washington of recruiting "forces from what it calls ‘the moderate opposition’… to undergo a training and rehabilitation programme run by the Central Intelligence Agency.

"A few days ago, one of these groups called Division 30 entered Syria… so Al-Nusra arrested several soldiers of this division," the group announced.

It did not specify how many rebels had been captured, or where or when the incident took place.

The Observatory had reported that Al-Nusra captured eight rebels from Division 30 in a village in the northern province of Aleppo late Wednesday.

Combined with the seven rebels killed on Friday, this would mean at least 13 of the 54 original Division 30 members who entered Syria in mid-July have been neutralised.

On Thursday, a US defence department spokeswoman denied the rebels’ reported capture.

Al-Nusra accused Division 30 of being "agents of American interests and projects in the area, to fight the ‘terrorist organisations,’ as they (the US and its rebel allies) described it." 

The jihadist group said Division 30 had coordinated with aircraft from the US-led coalition attacking jihadists in Syria to strike Al-Nusra positions.

"We warn the soldiers of this division not to proceed with the American plan… Their return to the truth and the right way will be more beneficial," the statement said.

Al-Nusra has previously attacked US-backed groups in northern Syria, including the Syrian Revolutionaries Front and the Hazm Movement, leading to the dissolution of both factions.