Beirut — Turkey has agreed to allow the United States to use Turkish soil to launch 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 was the culmination of nine months of negotiations between Washington and Turkey, which 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.

The details of the agreement were sealed in a telephone conversation Wednesday between President Barack Obama and Turkish President Recep Tayyip Erdogan, a senior U.S. administration official said, speaking on condition of anonymity to discuss sensitive information. A White House statement about the phone call said only that the two leaders had discussed "deepening our ongoing cooperation in the fight against ISIL, as well as common efforts to bring security and stability to Iraq and a political settlement to the conflict in Syria." The Islamic State is also known as ISIS and ISIL.

Turkey will allow the United States to fly armed drones and planes out of Incirlik, the official said. Incirlik is located just 60 miles from the Syrian border, and its use would enable U.S. warplanes to strike more quickly and efficiently against Islamic State targets in their northern Syrian strongholds, U.S. officials have said.

The Turkish government's previous refusal to allow the base to be used in the war against the Islamic State had triggered one of the deepest rifts in the U.S.-Turkish alliance in more than a decade, and it reflected deep-seated policy differences between Ankara and Washington over ways to address the Syrian war. Incirlik has hosted American forces under the umbrella of the NATO alliance for many years, but it remains subject to Turkish sovereignty.

In recent weeks, however, Turkey has showed signs of shifting closer to the U.S. position.

The agreement was reached amid heightened tensions between the Turkish military and the Islamic State along the Turkish-Syrian border. Hours earlier, Turkish forces clashed with Islamic State fighters near the border in their first significant ground engagement. Turkish troops fired artillery into Islamic State territory near the Kilis border crossing, killing two fighters, after Islamic State militants opened fire on Turkish troops guarding the area, according to Turkish media reports. At least one Turkish soldier was killed and two others were wounded, the reports said.

The shooting erupted after Turkey sought to prevent Islamic State fighters from entering illegally into Turkey via one of the many smuggling routes used to ferry goods, supplies and people in and out of Syria, Turkish media said. The Turkish military said in a statement that it scrambled four F-16 fighters to the area to guard against a possible escalation.

The U.S.-Turkish talks picked up speed in recent weeks as the Islamic State increased its presence close to the border in northwestern Syria. Elements of a broader deal also include the use of Turkish military ground spotters inside Syria to help guide the airstrikes, according to U.S. officials who were not authorized to publicly discuss the plan.

The arrangement would mark a significant escalation in Syria operations by the United States and its partners in an anti-Islamic State coalition. It comes as Islamic State forces, whose main activities have been in central and eastern Syria, have moved closer to Aleppo, the northwestern city that is Syria's largest.

Fighting in the northwest and in Aleppo has been primarily between Syrian opposition fighters and forces of President Bashar al-Assad's government, and the United States has been reluctant to use air power that it has said is solely devoted to the fight against the Islamic State.

Whether Turkey has secured any concessions from the United States regarding its own concerns was not immediately clear. Turkey has repeatedly said it wants Washington to focus as much on removing Assad as on fighting the Islamic State.

Under the plan currently being discussed, U.S. airstrikes would extend from Kobane, a Syrian town on the Turkish border, westward to the town of Azaz, about 20 miles north of Aleppo.

Turkey has long said that it wants a safe zone in the area, protected by air power, that would allow it to transfer back to Syria some of an estimated 2 million Syrian refugees on Turkish territory. It was unclear whether the U.S.-Turkey arrangement under discussion would recognize any safe zone, but increased control of the 560-mile border would enhance efforts to prevent Islamist militants from crossing into Syria.

Turkey had already been reinforcing its border in areas adjoining Islamic State territory with tanks and troops, and it recently imposed tighter controls on Syrians attempting to cross into Turkey.

Last week, Turkish police launched raids in Istanbul and other Turkish cities against suspected Islamic State facilitators engaged in smuggling foreign fighters into Syria, netting scores of suspects, according to Turkish media reports.

The latest fighting adds to concerns over militant designs in southern Turkey after a suicide bomber killed 32 people, many of them students, at a cultural center in the southern part of the country on Monday.

Turkish authorities say the suspected suicide bombing was likely the work of the Islamic State, which controls vast territory spanning northern Iraq and areas in eastern and northern Syria.