And the weapons they seized were most likely paid for with U.S. taxpayer dollars.

(Reuters) – Syrian rebels from an Islamist alliance formed last month have occupied bases and warehouses belonging to a Western-backed rebel group on the Turkish border, rebels and activists said on Saturday.

Fighters from the Islamic Front, a union of six major rebel groups, took control of the Free Syrian Army (FSA) bases at the Bab al-Hawa crossing on the northwestern border with Turkey late on Friday night, the opposition sources said.

Louay Meqdad, an FSA spokesman, said the Islamic Front fighters had entered the bases after saying they wanted to help to secure them. They then asked officers and employees to leave and replaced an FSA flag with one of their own, he said.

“We believe that those brigades are our brothers, that they know that we are not the enemy,” Meqdad said.

Infighting among Syria’s rebel groups has undermined their fight against President Bashar al-Assad in the 2-1/2-year-old civil war and made Western governments hesitant to back them.

The rise of hardline Islamist groups among the rebels, including some linked to al Qaeda, has also unsettled powers such as the United States, who fear that if the militants came to power, they would eventually turn their weapons on Western targets.

On Tuesday the Islamic Front said it had withdrawn from the military command of the FSA, notionally charged with coordinating the war, and criticized its leadership.