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.