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.
In recent days, IS fighters have advanced against the Syrian town of Kobane, which has a border crossing point with Turkey.
The US has been leading a coalition providing air support to Kurds battling IS fighters around Kobane.
Turkey has ranged its military forces on the border but has so far ruled out any ground operation on its own, and has refused to allow Kurds in Turkey to cross the border to fight.
Since the IS offensive against Kobane began in mid-September, some 500 people have been killed and up to 200,000 have fled across the border into Turkey.