How 400 US Troops Will Train Syria's Moderate Opposition to Fight ISIS

— -- Up to 400 American military trainers will begin this spring the next phase in the U.S. strategy against ISIS, training Syrian moderate rebels to fight the militant group inside Syria, Pentagon officials said today.

But which rebels will be trained by the U.S. military has yet to be decided as the Pentagon continues to develop a screening program that would ensure that trainees don't end up siding with extremist forces in Syria.

"Saudi Arabia, Turkey and Qatar have agreed to host training sites and we anticipate the program to train and equip the moderate Syrian opposition will take approximately 400 U.S. trainers," said Commander Elissa Smith, a Pentagon spokesperson.

The training program will be launched in early spring and the number of U.S. troops involved will grow as the trainers will also be accompanied by an undetermined number of support forces, officials said. Rear Admiral John Kirby, the Pentagon Press Secretary, told reporters that including the support forces, the number of American troops involved could reach 1,000.

Some of the additional support troops will provide security for the American trainers to protect them from an insider attack, Kirby said. “We've learned the hard way that you know, that's something we've got to be ever-vigilant on,” he said.

Since late 2012, the CIA has been training small groups of Syrian opposition fighters in Jordan, U.S. officials said, but the training program to be run by the U.S. military is much larger in scope.

Initial plans call for a program of 300 trainees per class with the expectation that 5,400 fighters will be trained in the following 12 months. Gen. Martin Dempsey, chairman of the Joint Chiefs of Staff, has said in congressional testimony that as many as 18,000 fighters may be needed inside Syria to have an impact against ISIS.

Each group will receive military training for 6 to 8 weeks before being reinserted into Syria, officials said, noting that the American trainers will most likely be special operations forces experience in training foreign militaries.

The training program is the last of the Obama administration's three-part strategy to counter ISIS in Iraq and Syria. The first step was an airstrike campaign to halt the extremist group's territorial advances inside Iraq and blocking its training and supply network inside Syria. The airstrikes were intended to buy time for the U.S. to restore Iraq's military capability to a point where they could launch a major offensive against ISIS -- the second part of the strategy.

These two parts of the "Iraq First" strategy have resulted in more than 1,821 airstrikes in Iraq and Syria and the presence of up to 3,100 American troops inside Iraq who will train, advise and assist Iraqi forces.

The third part of the plan was to carefully develop a training program where moderate opposition fighters would be trained in the region by American trainers. U.S. military planners have worked for months to find countries willing to host the training and develop a screening process to vet potential trainees.

While the administration's focus has been on developing a force that would fight ISIS inside Syria, Syrian opposition leaders have said that the training should be focused on fighting the military forces of the Assad regime. Turkey has made it clear that any fighters trained there will have no limitations on fighting the Assad regime.

But Pentagon officials have said that trained fighters will initially return to Syria in a defensive role to protect their home areas and provide stability for areas under opposition control. After that they will move into a more offensive role to fight ISIS, though there are no guarantees that they could not use their training against Assad's forces.

Precise details of when the training will begin are still unclear as the screening of recruits has yet to begin. According to Smith, once potential recruits are identified by name they will undergo a screening process that could last as much as 30 days.

"We've identified numerous groups that we believe are suitable for training based on our current understanding of the environment and we continue to evaluate the situation," Smith said.

Monitoring of trainees will continue inside Syria "once the trainees have returned to the battlefield in Syria. We cannot disclose the details of our sources and methods to conduct this monitoring," Smith said.