BEIRUT — Turkey has agreed to let the United States use Turkish soil to launch air attacks against the Islamic State, signaling a major shift in policy on the part of the once-reluctant American ally, U.S. officials said Thursday.

The decision to allow U.S. warplanes to use the Incirlik air base in southern Turkey is one element in a broad cooperation plan first broached nine months ago. Additional elements — including expanding U.S. airstrikes into the western part of the border area and using Turkish military ground spotters to guide them — are being discussed and finalized.

Turkey had resisted being drawn too deeply into the war against the Islamic State because of concerns about the direction of the Obama administration’s Syria policy.

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