The US effort to build what it calls "a moderate Syrian opposition force" that can allegedly stand up to the so-called ’Islamic State of Iraq and the Levant’ (ISIL) takfiri group is moving more slowly than expected.

The US effort to build what it calls "a moderate Syrian opposition force" that can allegedly stand up to the so-called 'Islamic State of Iraq and the Levant' (ISIL) takfiri group is moving more slowly than expected, the Pentagon said on Thursday.

Colonel Steve Warren, a Defense Department spokesman, said between 100 and 200 Syrian gunmen were undergoing training in the region, while hundreds more were still being screened or waiting to be brought out of the country.

Warren claimed that some 6,000 Syrians had volunteered to participate in the US-backed effort to train and equip the new armed units.

According to The Washington Post citing Pentagon officials, 4,000 - out of 6,000 - were waiting to be vetted. Some 1,500 have fully completed the screening process required to begin training.

"We have to identify Syrians who want to enter this program, we have to vet them, which is a difficult process," Warren told reporters at the Defense Department. However, Western media articles repeatedly reported that the gunmen operating in Syria against the national military are mercenaries coming from different world countries, mainly from communities of extremist ideologies.

"There's another difficulty, which is ex-filtrating these individuals out of Syria. ... Syria is a very complex, very dangerous place, multiple armed sides battling each other. So the ex-filtration process is also a significant challenge," he said.

Warren stressed that the training in Syria "will take as long as it takes based on the skill level that we see out of the trainees."

"We're going to ... continue the training until we get to a point where we believe that they're ready to rejoin the battlefield," he stressed.

Moreover, earlier reports about the CIA training programs for the terrorists operating in Syria raise questions about the credibility of the US-led declared military campaign against ISIL and other takfiri groups. The initial Pentagon plan called for the training of about 5,000 militants, but Martin Dempsey, Chairman of the US Joint Chiefs of Staff, said last year that it would take more like 12,000 to 15,000 gunmen.