STUTTGART, Germany -- U.S. warplanes involved in the fight against Islamic State militants in Iraq and Syria remained grounded at Incirlik Air Base for a second day on Sunday, raising concerns that a growing political dispute between Turkish and U.S. officials could jeopardize access to an operational hub that has become key to the U.S-led campaign.
On Saturday, Turkish authorities cut off commercial power to the base, which it owns and operates, and ordered the closure of airspace around Incirlik, hours after the government said it had gotten the upper hand over a group within the Turkish military that had attempted a coup on Friday night.
Some officials said the move was intended to ensure that no Turkish air assets loyal to the rebels were able to fly out of the base. But some analysts speculated that the move may be in part intended to pressure the U.S.
Turkish President Recep Tayyip Erdogan on Saturday asked the U.S. to extradite Turkish cleric Fethullah Gulen, whom Erdogan describes as the mastermind behind the failed overthrow. Gulen, who lives in exile in the U.S. and is an advocate for democracy and interfaith diaolgue, denied any involvement.
Secretary of State John Kerry said the U.S. would review any request that included "legitimate evidence."
Turkish Prime Minister Binali Yildirim said on Saturday that any nation supporting Gulen would be regarded as an "enemy" of Turkey.
Kerry in a call with Turkish Foreign Minister Mevlut Cavusoglu, cautioned that "insinuations" of American complicity in the attempted coup would have damaging political ramifications.
"He made clear that the United States would be willing to provide assistance to Turkish authorities conducting this investigation, but that public insinuations or claims about any role by the United States in the failed coup attempt are utterly false and harmful to our bilateral relations," State Department spokesman John Kirby said in a statement.
The dispute over Gulen threatens to further rock an already troubled relationship between Washington and Ankara, and potentially puts operations at Incirlik in the political balance.
U.S. fighters, drones and refueling aircraft that have played a crucial role in the battle against the Islamic State group from Incirlik are on standby. As of Sunday morning, commercial power remained cut off at Incirlik, forcing the base to rely on generators. The airspace around Incirlik also remained closed, U.S. European Command said Sunday.
Since August, A-10 Warthogs and other aircraft have been a constant presence at Incirlik, expanding the U.S. coalition's reach into Iraq and Syria. Prior to being able to fly out of Incirlik, the U.S. had relied on bases in the Middle East and carriers in the Persian Gulf to conduct operations. Turkey had initially rejected U.S. requests when the campaign against the Islamic State began in 2014 to conduct operations from Incirlik. But in the summer of 2015, Ankara reversed course. For pilots, that meant shorter flights to targets in Islamic State strongholds in northern Syria, where A-10s have played a prominent role .
During a December visit to Incirlik, Defense Secretary Ash Carter called the airmen leading the effort out of Incirlik the "tip of the spear."
"You are first and foremost, and I'll come back to this, hastening the defeat of ISIL, which has to occur and will occur, and this is a very important location in the point of the spear in that campaign," Carter said, using an acronym for the Islamic State group.
With shorter flight times and air-refueling capabilities at Incirlik, pilots have been able to spend more time in the air, giving commanders a clearer picture of conditions on the ground. Flights out of Incirlik are responsible for an estimated one-third of all air refueling operations.
Given Incirlik's status in the fight, the loss of access to the base could require a reorganization of how assets are allocated and used in the campaign against the Islamic State.
The U.S. also has a small presence in Diyarbakir, a staging sight for an air force search and rescue mission in southern Turkey. The U.S. and Turkey also have plans to position an American High Mobility Artillery Rocket System in southern Turkey to assist operations in Syria.
The status of those plans is unclear.
With the current halt to flights at Incirlik, the U.S. coalition has been forced to adjust its operations, according to the Pentagon. On Saturday, the number of strikes launched against Islamic State targets was 17 compared to the previous day's 23.
U.S. Central Command downplayed the effect of the halt on flights out of Incirlik on operations overall.
"While Incirlik (Air Base) provides a convenient location, it is important to remember that coalition aircraft operate from multiple other locations throughout Southwest Asia in support of counter-ISIL air operations, to include from U.S. naval assets in the region," CENTCOM said in a statement. "U.S. officials are working with the Turks to resume air operations at Incirlik as soon as possible, but in the meantime, U.S. Central Command is adjusting flight operations elsewhere within the theater of operations to minimize any effects on the overall counter-ISIL campaign."
For the U.S., the relationship with NATO ally Turkey has been fraught with complications. On the one hand, U.S. officials describe Turkey as a "stalwart" ally. On the other, Erdogan is viewed as a leader who over the past several years has demonstrated increasing authoritarian tendencies as he consolidates control in the country, where journalists have been imprisoned and political opponents rounded up.
Turkey also has a history of making demands on NATO allies.
For example, when Turkish airspace was violated in 2013 by errant shells fired by forces connected to the Assad regime in Syria. Ankara called an emergency meeting of allies in NATO. The result: allies agreed to send three Patriot missile batteries to Turkey, including one from the U.S. whose soldiers guarded Turkish airspace around the clock for two years.
The multimillion dollar deployment ended for the U.S. in 2015, and during that nearly two year span not a single Patriot was fired in defense of Turkish territory.
-- Stars and Stripes reporter Chris Church contributed to this report.