In this file image posted on a Twitter account of Syria's al-Qaeda-linked Nusra Front on Tuesday, July 7, 2015, a fighter from the Nusra Front fires a weapon during clashes against the Syrian government forces at the western Zahra neighborhood in Aleppo city, Syria.(Photo: AP)

U.S.-trained rebels handed over American-supplied vehicles and ammunition to an al-Qaeda linked group shortly after arriving in Syria, the Pentagon said Friday, the latest blow to a Pentagon program that has been plagued by problems since its inception.

The rebels surrendered six pickup trucks and ammunition to an Nusra Front “intermediary,” the Pentagon said in a statement.The Nusra Front is an al-Qaeda affiliate in Syria.

The equipment was about one-quarter of what the group had been issued and was turned over the Nusra Front in exchange for safe passage through area controlled by the al-Qaeda affiliate.

The allegations, if true, would be "a violation of Syria train and equip program guidelines," Col. Patrick Ryder, a spokesman for U.S. Central Command, said in the statement.

Pentagon officials initially denied media reports this week that the U.S.-backed rebels had turned over arms or equipment to the al-Qaeda affiliate. But on Friday they learned some of the allegations were true after U.S. backed rebels acknowledged it.

"We are using all means at our disposal to look into what exactly happened and determine the appropriate response," Ryder said.

The plan is part of an effort to vet Syrian rebels to form a ground force to combat the Islamic State. The rebels would be trained by U.S. advisers in locations outside the country and then inserted back into Syria.

The initial plan called for training more than 5,000 rebels per year. It hasn't met expectations.

The first group of about 50 fighters entered Syria in July but came under immediate attack. Most of the initial force disbursed or was killed, leaving only a handful to fight.

On Monday, the U.S. military said another 70 fighters entered Syria to fight the Islamic State. They handed over the equipment almost immediately after entering Syria.

In recent testimony, Gen. Lloyd Austin acknowledged that the program, which began training in May, is moving slower than anticipated. The Pentagon has said it is examining other options for building a ground force in Syria to combat the Islamic State.

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