NEW YORK - The US is struggling to implement its training programme for moderate Syrian rebels fighting the Islamic State (IS) group, according to figures released Thursday by the Pentagon.

Only "100 to 200" fighters have actively begun training at US 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 programme 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.

Slow-downs have occurred in the vetting of trainees. Some 6,000 Syrians have volunteered for training, and 1,500 passed the first stage of selection.

The US-trained Syrian fighters would be equipped with pickup trucks with mounted machine guns, radios and global positioning system trackers, reported Reuters.

The US military is planning to send more than 400 troops, including special operations forces, carry out the training.

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 the IS group 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 programme is also going up against strategic difference between Washington and regional partners such as Turkey.

Ankara has claimed Washington's singular focus on the IS group weakens the fight against Syrian President Bashar al-Assad's regime.

The US is experiencing a similar training shortage in Iraq, where Carter said yesterday that Baghdad hasn't sent nearly enough forces to US training facilities to help prepare its forces and retake areas held by IS fighters.