Al-Qaeda's Syria branch has posted a video purportedly showing its capture last week of members of a US-trained rebel force it accuses of aiding US-led air strikes against its fighters.

The Pentagon denied on Thursday that any graduates of its training programme for moderate rebels had been captured in Syria.

But the Syrian Observatory for Human Rights said that eight members of the 54-strong Division 30 unit inserted into Aleppo province in mid-July were being held by Al-Qaeda affiliate Al-Nusra Front.

The video released by Al-Nusra on its YouTube account on Saturday appears to show at least some of the captured rebel fighters.

It depicts five men walking through a field in a straight line, hands behind their heads, supervised by one hooded man and one armed man.

One of the apparently detained men tells the camera he was recruited by the US, through intermediaries, to receive training in Turkey for a month and a half.

He said the trainees were each given an M16 assault rifle and some cash to "fight Al-Nusra" in Syria.

A hooded man identifying himself as an Al-Nusra member said the jihadist group had "cut the hand of the West and the Americans in Syria" by capturing the men.

"Their collaboration with the West is clear," he said, accusing the detained men of helping US-led air strikes against Al-Nusra positions.

While the Islamic State group has been the main focus of the coalition bombing campaign, Al-Nusra too has been targeted despite its fierce hostility to its jihadist rival.

In an online statement it published on Friday, Al-Nusra called Division 30 "agents of American interests and projects in the region."

The same day, it launched an offensive against Division 30's headquarters.

In a statement on Facebook, the US-trained rebel group said five of its fighters had been killed and 18 wounded in the battle for the base.

Under the cover of US-led air strikes, the remaining forces withdrew to the Afrin area of Aleppo province, which is held by Kurdish militia.

At least 25 Al-Nusra fighters were killed, the Observatory said.