Turkey has agreed to let the US use its military bases in the campaign against Islamic State militants, US National Security Adviser Susan Rice says.
Ms Rice said the US welcomed the new agreement, which included use of the Incirlik air base in Turkey's south.
The US is leading air strikes against IS, which has seized large parts of Iraq and Syria in recent months.
Turkey shares a border with both countries, but has so far ruled out any ground operation of its own.
Speaking to US broadcaster NBC, Ms Rice said Turkey had recently agreed to let the US use Turkish bases and territory "to train moderate Syrian opposition forces" and "engage in activities inside of Iraq and Syria".
"That's the new commitment, and one that we very much welcome," she added.