The program to train moderate Syrian rebels could involve as many as 1,000 U.S. trainers and support personnel, the Pentagon said Friday.

If all goes well, trained rebels could be back on the battlefield in Syria by the end of the year.

Rear Adm. John Kirby said the U.S. will send several hundred trainers and probably an equal number of support troops, including security teams, to the training sites, which will be in Turkey, Saudi Arabia and Qatar. The exact number of U.S. troops to be sent on the mission has not been finalized, he said, but it could top 1,000.

"All told, the number total for this mission could approach 1,000," he said. "It might even exceed that. I can't rule that out."

There also could be hundreds of trainers from other countries, said Kirby, including troops from nations hosting the training sites as well as other nations.

The opposition forces are to be trained for three main purposes, Kirby said: to defend their own towns inside Syria against extremist groups like the Islamic State group, to later go on the offensive against IS forces, and eventually to work toward a political solution to the Syrian civil war.

Kirby said the U.S. is now attempting to identify Syrian moderate groups from which to recruit. "But no recruiting has actually yet started. No trainees have been identified and enrolled in this program just yet," he said.

Plans for the U.S. and others to train Syria's moderate opposition have dragged on for months as officials have struggled to work out details of the program and put together a vetting process to identify appropriate rebel groups and individuals who could receive the training.

Kirby said the training could begin this spring, and it will take several months to complete the first round of training, which he said will be conducted by a mix of U.S. special operations forces and conventional U.S. troops.

"If the training is able to start in March, you could be looking at some opposition groups ... getting back into Syria and into the fight ... before the end of the year," Kirby said. "I think that's certainly a possibility, but we've got a lot of work to do before we're there."

Kirby said the vetting will be done in phases over the course of the training program, to insure that trainers are dealing with units and individuals that are trustworthy and reduce the risk of rogue attacks on the U.S. trainers.

Last month, President Barack Obama signed into law a massive defense policy bill that endorsed his plan to fight Islamic State militants, including air strikes and training of Iraqis and moderate Syrian rebels. The law authorized the training and equipping of moderate Syrian rebels battling the extremists for two years, and provided $5 billion to train Iraqis battling the militants who brutally rule large sections of the two countries.