The Pentagon's effort to train and arm so-called moderate Syrian rebels to fight against the Islamic State group (ISIS) has experienced a number of setbacks, officials revealed Thursday.

Less than 200 of the 6,000 Syrians who volunteered to be part of the program have begun training, Pentagon spokesman Col. Steven Warren told CNN.

Warren said one of the main problems delaying the program has to do with the "exfiltration" of qualified fighters - that is, the risky process of getting the Syrian volunteers out of their native country and into either Turkey or Jordan, where American forces have multiple training sites. As of yet, none of the rebels have completed training.

Approximately 4,000 of the volunteer rebels are still waiting to be fully vetted by the Defense Department, which is a critical step to ensure they won't later defect to ISIS or another radical group along with their U.S.-provided weapons and training, as has occurred multiple times in the past.

The Pentagon says they also want to make sure that the rebels will focus their efforts on fighting against ISIS, rather than the Assad regime. Last September, rebels in the Free Syrian Army group, which the U.S. previously armed, vowed that they would continue using U.S. weapons to fight the Assad regime as well as ISIS, The Daily Beast reported.

So far, about 1,500 of the 6,000 have passed the Pentagon's pre-screening and are awaiting training, while 500 have been turned away for various reasons, according to CNN. The goal was to train 3,000 over the remainder of 2015, and 5,400 each succeeding year.

Congress approved $500 million in December for the program, The Hill reported.

"Syria train-and-equip program is even more challenging than the Iraq train-and-equip program," Defense Secretary Ash Carter told lawmakers at a House Armed Service Committee hearing on Wednesday. "We are trying to recruit people that ... can be counted on - that is, to fight, to have the right mindset and ideology. It turns out to be very hard to identify people who meet both of those criteria."

The U.S. is still mulling over what responsibility it will take for the fighters once they are armed, trained and sent back to their home to fight ISIS.

"I believe we have some obligation to support them and protect them, including supply them," Carter said, noting that there is concern that U.S.-provided weapons could be "diverted" to ISIS.

"These constraints that we put on ourselves, which are perfectly understandable, do progressively limit the number of inductees into the program," he said, according to CNN. "And that's proving (to be) the thing that limits the growth of the program. We have enough training sites and so forth for them. For now, we don't have enough trainees to fill them."

The Pentagon program operates in addition to a separate and extremely secretive CIA program to train and arm Syrian rebels for an entirely different purpose: overthrow Syrian President Bashar al-Assad.

The House Intelligence Committee recently voted to unanimously cut as much as 20 percent of that black budget program, which has become one of the agency's largest covert operations, training and arming nearly 10,000 Syrian fighters in the past few years, The Washington Post reported. The program's budget was for the first time recently revealed to be around $1 billion a year - roughly $100,000 per year for each anti-Assad rebel receiving training.