ANKARA — The Turkish cabinet has signed a decree that allows the US and ally militaries to use a critical air base in southern Turkey for airstrikes against Islamic radicals in Syria, a senior Turkish diplomat said.

Turkish Foreign Ministry Spokesman Tanju Bilgic said Wednesday that the Incirlik air base would be used for airstrikes as part of a non-written agreement between Turkey and the US.

"The air base can be opened for use [air strikes] any moment," he said. "The primary target is the Islamic State of Iraq and Syria (ISIS)."

The Turkish green light came at a time when the US is planning to expand its military footprint in Turkey with more personnel and combat aircraft. "We're talking about logistics … berthing and force protection and things like that," Navy Capt. Jeff Davis, a Pentagon spokesman, said July 27.

NATO allies US and Turkey aim to create an ISIS-free zone in northern Syria. ISIS has captured large swaths of land in Syria and Iraq, both neighboring Turkey, since last summer.

US officials think Incirlik Air Base in southern Turkey is close to many Islamic State strongholds, and expanding US operations there would cut response times for US aircraft and increase the efficiency of anti-militant operations.

About 1,700 US service members are at Incirlik, according to Defense Department data.

Turkey in the past imposed several restrictions on the use of Incirlik by the US military. But Ankara decided to more broadly cooperate with the US-led alliance fighting ISIS after a suicide bomber belonging to the jihadist group killed 32 pro-Kurdish activists in a Turkish town bordering Syria on July 20.