Only four or five U.S.-trained Syrian rebels were still fighting in Syria, a top general told Congress today, a stark admission of setbacks to a fledgling military program that critics have already pronounced a failure.

The U.S. military began training in May for up to 5,400 fighters a year, in what was seen as a test of President Barack Obama's strategy of having local partners combat Islamist militants and keep U.S. troops off the front lines.

But the program was challenged from the start, with the first class of less than 60 fighters coming under attack from al Qaeda's Syria wing, Nusra Front, in their battlefield debut. Some were captured and killed while others scattered.

U.S. General Lloyd Austin, the head of the U.S. military's Central Command, told the Senate Armed Services Committee that at the current, slower-than-expected pace, the initial training targets would be unreachable.

Austin acknowledged the program was under review but he said the numbers of Syrian fighters would grow over time.

Asked how many fighters were still in Syria, Austin said: "It's a small number. The ones that are in the fight ... we're talking four or five."

Under Secretary of Defense for Policy Christine Wormuth told the committee that only 100 to 120 Syrian fighters were in training.