The U.S. military on Saturday was working with officials in Turkey to get a key air base operational in the fight against the Islamic State.

Security was boosted to its highest level at bases across Turkey during Friday’s coup attempt against the Turkish government, a measure typically reserved for when there is an ongoing terrorist attack.

A U.S. defense official, speaking on the condition of anonymity to discuss ongoing operations, said Saturday that force protection was elevated Friday night to level Delta. That typically signifies a terrorist attack has occurred or that one is considered imminent nearby.

Pentagon Press Secretary Peter Cook said Saturday that the Turkish government’s decision to close its airspace to military aircraft resulted in the halting of air operations at Incirlik Air Base, in the southern part of the country near Syria.

“U.S. officials are working with the Turks to resume air operations there as soon as possible,” Cook said. “In the meantime, U.S. Central Command is adjusting flight operations in the counter-ISIL campaign to minimize any effects on the campaign. U.S. facilities at Incirlik are operating on internal power sources and a loss of commercial power to the base has not affected base operations.”

The Pentagon has increasingly relied on Turkish military installations as the United States has waged its war against the Islamic State in Iraq and Syria. In particular, U.S. troops at Incirlik and at Diyarbakir Air Base in the southeastern part of the country both have a direct role in the military campaign against the militants.

At Incirlik, the United States has based A-10 attack planes, KC-135 tankers and unmanned aircraft. An Air Force commander there said in May that his unit handled one-third of all refueling operations for the air war over Iraq and Syria.

An A-10 squadron has been based at Incirlik since October 2015, after the Turkish government allowed U.S. strike aircraft to use the base. Use of the base immediately improved how long the aircraft could remain over Iraq and Syria, considering its proximity when compared with other military bases used by the Pentagon in Persian Gulf countries.

At Diyarbakir, the Air Force began staging small numbers of U.S. troops last fall in case personnel-recovery missions were required. The installation is home to Turkey’s 8th Air Wing and was not planned as a permanent home for U.S. troops, U.S. military officials have said.

Senior defense officials also said in April that they were planning to place a mobile rocket system known as HIMARS, short for High Mobility Artillery Rocket System, somewhere in Turkey to support U.S. operations in Syria.

In other parts of Turkey, U.S. troops use an air base in Izmir, some 300 miles southwest of Istanbul, and Aksaz Naval Base, on the Aegean coast. In March, the Pentagon and State Department ordered families of U.S. troops and diplomats to leave the country, citing security concerns raised by terrorist attacks across Turkey.

It was not immediately clear what increased security measures were taken at U.S. diplomatic facilities in Turkey. A statement released Saturday by the U.S. Embassy in Ankara warned that the airport in Istanbul still wasn’t safe to use.

“We urge U.S. citizens to contact family and friends to let them know you are safe,” embassy officials said. “We have seen reports that social media is blocked, but you can contact friends and family by email, telephone or SMS.  We encourage U.S. citizens to shelter in place and do not go the U.S. Embassy or Consulates at this time.”

But the defense official said the State Department has not requested the U.S. military to reinforce any of its diplomatic facilities in Turkey, which include the embassy in the capital of Ankara and consulates in Istanbul and Adana. No Americans were flown out of Turkey by the U.S. military during the coup attempt, the official added.

This story has been updated multiple times.

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