WASHINGTON — Moving from reluctance to refusal and finally to acceptance, Turkey is joining its NATO allies and fellow Sunni Muslim nations in a coalition to destroy the Islamic State militant group. But the U.S. is still waiting for details of any new, specific aid and is warily watching to make sure Ankara keeps its commitments.

Turkey, often described as a difficult partner with much of the West, long has resisted being used as a launching pad for foreign troops to attack neighboring nations. Now, however, Turkey faces threats from militants who have overtaken much of northern Syria and Iraq, as well as from Kurdish separatists whom Ankara considers terrorists.

And while Turkey has been overwhelmed by an estimated 1.5 million Syrian and Iraqi refugees, it also has been accused of harboring — or at least helping — Islamic State militants by letting them move weapons and oil over its borders.

State Department officials Wednesday said Turkey has yet to pledge any specific assistance. A day earlier, Secretary of State John Kerry confidently said “Turkey is very much part of this coalition and Turkey will be very engaged on the front lines.”