The U.S. effort to build a moderate Syrian opposition force that can stand up to the self-proclaimed Islamic State of Iraq and al-Sham (ISIS) militants is moving more slowly than expected due to complications vetting volunteers and bringing them out of Syria for training, the Pentagon said on Thursday.

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

"As of now, none have completed training," Warren said.

"We are certainly below our expectation on throughput," he added. "As far as recruits for the Syrian train-and-equip mission, we're satisfied. It's the final step that we're having difficulty (with)."

He said some 6,000 Syrians had volunteered to participate in the U.S.-backed effort to train and equip a politically moderate Syrian military force. Of that number, 4,000 were waiting to be vetted. Some 1,500 have fully completed the screening process required to begin training, Defense officials said.

"There are challenges," Warren told a briefing. "This is a very difficult operation to undertake. We have to identify Syrians who want to enter this program, we have to vet them, which is a difficult process."

"There's another difficulty, which is exfiltrating these individuals out of Syria. ... Syria is a very complex, very dangerous place, multiple armed sides battling each other. So the exfiltration process is also a significant challenge," he said, calling it a "choke point."

About 5,000 Syrian fighters are targeted to be trained per year for three years under the program, through sites offered by Jordan, Turkey, Saudi Arabia and Qatar.

The train-and-equip program for moderate Syrian opposition forces has already kicked-off in Turkey and Jordan, whereas logical preparations are currently underway for it to start in Saudi Arabia and Qatar.

Turkey, however, has been continuously saying that a more comprehensive strategy --along with training of the opposition groups-- is need to find a permanent solution to Syrian war. Accordingly it has been suggesting that safe and no-fly zones be established in Syria.

Turkey has continuously stressed that establishing a safe zone is a must and providing air protection to those trained individuals is highly crucial.