The United States has said that a group of rebel fighters it trained and sent to Syria have handed US-supplied vehicles and ammunition over to the local branch of al-Qaeda, al-Nusra Front.

"Today the NSF [New Syrian Front] unit contacted coalition representatives and informed us that on September 21-22 they gave six pick-up trucks and a portion of their ammunition to a suspected al-Nusra Front intermediary, which equates to roughly 25 percent of their issued equipment," according to Central Command spokesman, Colonel Patrick Ryder.

"If accurate, the report of NSF members providing equipment to al-Nusra Front is very concerning and a violation of Syria train and equip programme guidelines," Ryder said.

The admission, after previous denials, is the latest in a series of blows to the 500-million-dollar US programme to establish a Syrian rebel force capable of fighting the Islamic State extremist militia on the ground.

Friday's admission is a complete reversal from earlier in the week when the U.S. military denied reports that the newly trained unit had turned over the equipment.

Some 75 US-trained fighters entered Syria last week and took up positions on frontlines against ISIS in Aleppo province, according to the Syrian Observatory for Human Rights.

They were the second group of fighters of the planned New Syrian Force to cross the border from Turkey.

A previous group of 54 entered Syria in July but came under attack from al-Nusra. Central Command chief General Lloyd Austin last week said only four or five of that group were still operating inside Syria.