WASHINGTON - The US military is planning to deploy more than 400 troops to help train Syrian rebels to fight the Islamic State, along with hundreds of US support personnel, a Pentagon spokesman said on Thursday.

The US military has not yet identified where it will draw its forces from for the training mission, expected to begin in the spring at sites outside Syria, Colonel Steve Warren said. Turkey, Qatar and Saudi Arabia have offered to host the training.

Warren did not offer additional details on the troop figures, first reported by Defense One website.

The training program is a part of President Barack Obama's multi-year plan to field local forces in Syria to halt and eventually roll back Islamic State fighters, while pounding them with US-led airstrikes.

The Pentagon has estimated that it can train more than 5,000 recruits in the first year and that up to 15,000 will be needed to retake areas of eastern Syria controlled by Islamic State.

Critics in Congress have said the Pentagon program won't aid Syrian opposition forces fast enough, however, and question whether it is too small to influence the course of Syria's multi-pronged civil war between President Bashar al-Assad and his opponents.

Across the border in Iraq, Obama has authorized more than 3,000 US troops to advise and train Iraqi and Kurdish forces.

The disclosure of the planned troop deployments for the Syria training mission came just days after senior US officials met Syrian opposition and civil society leaders in Istanbul to discuss the program.

US Major General Michael Nagata, Combined Joint Interagency Task Force - Syria Director, and US Special Envoy for Syria Daniel Rubinstein led the meeting on the US side.

"These introductory meetings were an important step as we prepare to launch the train-and-equip program later this spring with our international partners," said Pentagon spokeswoman Commander Elissa Smith.