Turkey is to allow US warplanes to launch airstrikes against the so-called Islamic State group from Incirlik Air Base in the south of the country, US officials said.
The move comes after months of negotiations and was revealed a day after US President Barack Obama spoke with his Turkish counterpart Recep Tayyip Erdogan about the conflict.
"Access to Turkish bases such as Incirlik air base will increase the coalition's operational efficiency for such counter-ISIL efforts," a defence official said.
Word of the deal also came as the Turkish military hit IS militants on the Syrian side of the border, in a drastic escalation of the conflict.
It followed the killing of a Turkish soldier in cross-border fire from IS militants.
The clash in Syria was the most serious between the Turkish army and IS since the militants began to take swathes of Iraq and Syria right up to the Turkish border from 2013.
Turkey shares a 800km border with Syria, and a section of its southern frontier bounds territory controlled by the IS group.
The United States operates both manned and unmanned aircraft in its bombings of IS targets, but had previously not been permitted to use its facilities on bases in its NATO ally Turkey.
US Defense Department spokesperson Laura Seal said: "We have decided to further deepen our cooperation in the fight against ISIL, our common efforts to promote security and stability in Iraq, and our work to bring about a political settlement to the conflict in Syria."
On Monday, 32 people were killed in a suicide bombing in a Turkish town on the Syrian border that was blamed on IS and sparked an upsurge in violence in Turkey's Kurdish-dominated southeast.
"As allies, we take threats to Turkey's border very seriously," said Ms Seal.
"We remain committed to Turkey's defence and will work with Turkey to deepen our cooperation against the shared threat of terrorism."
Turkish police raid suspected IS, PKK militants in Istanbul: reports
Meanwhile, Turkish police launched raids to arrest suspected members of IS and Kurdish militants, the official Anatolia news agency has said.
Backed up by helicopters, police raided addresses in several Istanbul districts in search of members of IS, the Kurdistan Workers Party (PKK) and other militant groups, it added.
The prime minister's office said in a statement that 251 people had so far been detained in the simultaneous raids.
