The United States is struggling to implement its training program for moderate Syrian rebels fighting the Islamic State (ISIS) group, the Pentagon admitted Thursday, according to AFP.

Only “100 to 200” fighters have actively begun training at American sites in Jordan and Turkey, according to Pentagon spokesman Colonel Steve Warren, of the 5,000 forces the military says it wants to train over the year.

The Obama administration embarked on the program earlier this year under strong pressure from Congress, which allocated $500 million to finance it.

“This is a very difficult operation to undertake,” Warren said.

Last month it was reported that the United States military had begun training a small unit of “moderate” Syrian rebels in Jordan to return home and take on ISIS, a process which began after months of intense vetting.

Slow-downs have occurred in the vetting of trainees, however, according to Thursday’s report. Some 6,000 Syrians have volunteered for training, and 1,500 passed the first stage of selection, according to the report.

Pentagon Chief Ashton Carter told a congressional committee Wednesday that it is difficult to find fighters that are both moderate and willing to take on ISIS as their primary foe.

“It turns out to be very hard to identify people who meet both of those criteria,” Carter said.

Besides difficulties of implementation, the training program is also going up against strategic difference between Washington and regional partners such as Turkey.

Ankara has claimed Washington’s singular focus on ISIS weakens the fight against Syrian President Bashar Al-Assad’s regime.

The U.S. is experiencing a similar training shortage in Iraq, where Carter said on Wednesday that Baghdad hasn’t sent nearly enough forces to training facilities to help prepare its forces and retake areas held by ISIS jihadists.

AFP contributed to this report.