A senior Turkish official said that Ankara threatening to close down the U.S. Air Force Incirlik Air Base.

Turkish Foreign Minister Mevlut Cavusoglu said in a statement that Ankara may insist that the U.S. leave Incirlik air base if Washington goes ahead with the sanctions it has threatened in response to Turkey’s purchase of S-400 air defense systems.

“We will assess the worst-case scenario and make a decision. If the US imposes sanctions against Turkey, then the issue of the Incirlik and Kurecik bases may be on the agenda,” Cavusoglu said.

Deliveries of the latest Russian-made S-400 air defense systems, which caused a significant rift in relations between Turkey and the United States, began in July. According to Turkish President Recep Tayyip Erdogan, the S-400 will be fully operational in April 2020.

The vast Incirlik Air Base, located in southern Turkey close to Syria, has been a longstanding symbol of U.S.-Turkish cooperation. At the height of the Cold War, it underscored America’s commitment to its NATO partner against the Soviet Union.

Incirlik Air Base has a U.S. Air Force complement of about five thousand airmen, with several hundred airmen from the Royal Air Force and Turkish Air Force also present. The primary unit stationed at Incirlik Air Base is the 39th Air Base Wing (39 ABW) of the U.S. Air Force. Incirlik Air Base has one 3,048 m (10,000 ft)-long runway, located among about 57 hardened aircraft shelters.

It is worthwhile noting that estimated B61 nuclear gravity bombs stored at İncirlik airbase, which is about 100 miles from the Syrian border and which the US air force shares with its Turkish counterpart.

Of the five nuclear weapons storage locations in Europe, Incirlik Air Base in Turkey stores one-third of the weapons in Europe, although there are unconfirmed rumors that the weapons may have been withdrawn.