Turkey agreed to allow the U.S. to use air bases there to launch strikes against Islamic State forces in neighboring Syria, a major shift long sought by Washington and sealed hours before a deadly clash between Turkish forces and militants across the border.

After months of tense negotiations, reluctant Turkish leaders agreed to U.S. requests to use the Incirlik Air Base in southern Turkey to send fighter jets and armed drones to attack fighters based in Syria and Iraq, U.S. defense officials said.

Incirlik is a U.S. base about 60 miles from the Syrian border, giving U.S. jets and unmanned drones vastly improved logistics for daily attacks on the radical group compared with other sites across the region now in use. Military officials said the agreement also opens up other bases in Turkey for potential use in the campaign against Islamic State. […]