The US Department of Defense is to send 400 troops and hundreds of support staff to train moderate rebels against Islamic State (IS) in Syria.
It is not yet clear where the troops will be drawn from or where they will be based, though Turkey, Qatar and Saudi Arabia have offered to host them.
The US aims to train more than 5,000 rebels annually for three years.
It marks an expansion in the US training of Syrian rebels which began in March 2013 in Jordan.
This covert programme was led by the CIA. In October 2014 it was announced that the project would be escalated and a parallel Pentagon programme established.
The latest announcement comes days after senior US officials met Syrian opposition leaders in Istanbul, Turkey.
The US hopes that by training the rebels, they will be more effective in the fight against IS militants.
Across the border in Iraq, the US has sent more than 3,000 troops to train Iraqi and Kurdish forces.
Critics of the training mission say it is too small and too slow to be effective against IS. The CIA estimates that the group may have as many as 31,500 fighters.
There are also concerns that the US may end up training rebels linked to other extremist groups such as the al-Qaeda affiliate the Nusra Front.
The US has been carrying out air strikes against IS in Iraq and Syria since August as part of an international coalition to "degrade and destroy" the group.