The Pentagon is planning to send 400 troops and hundreds more supporting personnel to train rebels fighting the Islamic State in Syria.
The personnel will be deployed in three countries around Syria in March, as part of US efforts to support rebels against IS.
The announcement comes a few days after senior officials met with Syrian opposition leaders in Istanbul.
Defence officials told the Defense One website that they recognise the challenges they face with the training program for rebels in Syria.
“This is going to be hard,” one senior defence official said.
“We have to recruit the guys, we have to assume that there are a lot of guys who are recruitable, there’s got to be some vetting. This is not going to be an easy enterprise here.”