The United States has said that a group of rebel fighters it trained and sent to Syria have handed U.S.-supplied vehicles and ammunition over to the local branch of Al-Qaida.

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

"Unfortunately, we learned late today that the NSF (New Syrian Forces) unit now says it did in fact provide six pickup trucks and a portion of their ammunition to a suspected Al-Nusra Front (group)," Pentagon spokesman Captain Jeff Davis said.

U.S. Central Command spokesman Colonel Patrick Ryder said that members of the rebel unit contacted U.S. forces on Friday and informed them that they had given equipment to "a suspected ... intermediary" of the Nusra Front, the Syrian branch of Al-Qaida.

The equipment, about a quarter of what had been issued to the rebels, was handed over "purportedly in return for safe passage within their operating area," a Central Command statement said.

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

Some 75 U.S.-trained fighters entered Syria last week and took up positions on frontlines against Islamic State 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 Nusra Front. Central Command chief General Lloyd Austin last week said only four or five of that group were still operating inside Syria.

U.S. officials have recently played down the importance of the rebel training program, pointing instead to the successes of Syrian Kurdish forces, which have made considerable gains this year against the jihadists with U.S.-led air support.

The setbacks for the U.S. rebel training program come as Russia ramps up its military assistance to Syrian President Bashar Assad, claiming that his forces are the only ones capable of taking on Islamic State on the ground.