The United States has said it will send 400 troops and hundreds of support personnel to train moderate Syrian rebels fight against the Islamic State of Iraq and the Levant (ISIL).

The Pentagon confirmed the planned deployment early on Friday, after Defense One website reported that Turkey, Qatar and Saudi Arabia offered to host the training.

The US military estimated it could train more than 5,000 recruits in the first year. It said up to 15,000 would be needed to retake areas of eastern Syria controlled by ISIL.

The announcement came just days after senior US officials met Syrian opposition and civil society leaders in Istanbul to discuss the programme.

Syria's neighbour Jordan, already hosts a small and ostensibly covert, effort by the CIA to equip and train small groups of fighters opposed to the Syrian President Bashar al-Assad.

The latest training programme is a part of US President Barack Obama's multi-year plan to halt ISIL, including air strikes and training Iraqis and moderate Syrian rebels.

Critics in Congress said the Pentagon programme would not aid Syrian opposition forces fast enough, and question whether it was too small to influence the course of Syria's multi-pronged civil war between President Assad and his opponents.

The Syrian civil war, which began as a peaceful uprising against Assad's rule more than three years ago, has become a bloody and protracted sectarian conflict killing more than 200,000 people.

ISIL has seized large swathes of land in Syria and neighbouring Iraq, and battles both regime forces and rebel groups for control of territory.

In June last year, it announced the establishment of a "caliphate" straddling the two countries and the Syrian city of Raqqa as its headquarters.