Washington (CNN) The US military is doing "prudent planning" in case it needs to move its anti-ISIS operations out of Incirlik Air Base in southern Turkey following the country's failed coup, several US defense officials told CNN Tuesday.

The Turkish government cut off the power to the base after the coup attempt took place Friday, leaving it without a commercial power supply for four days and counting.

The Pentagon has been able to fully operate the base through generator power but said it would be difficult to continue indefinitely.

While fuel supplies for the generators could be brought in, the key is whether it becomes too expensive and cumbersome given the high pace of air strikes the US wants to continue flying out of the base. US aircraft regularly conducts missions in northern Syria, where it is battling ISIS. And both drones and surveillance aircraft are used to monitor the border with Syria and conduct other reconnaissance missions.

The Pentagon is all but openly pressuring the Turks to turn the power back on at Incirlik.

If they don't, "I think it's safe to say over time that it could become a limiting factor," said Peter Cook Pentagon press secretary, referring to US operations there. "The concern would be if it were a protracted period of time, then we would potentially have to make adjustments."

The critical value of Incirlik is its location in southern Turkey, which allows the US to conduct a high volume of missions because it doesn't require the extensive refueling that is needed when aircraft come from bases further away in the Persian Gulf.

Moving the aircraft out of Turkey would mean relocating more than a dozen manned and unmanned aircraft plus dozens of personnel to another country, US defense officials said, but did not disclose where they might go. Placing some equipment in Italy or Iraq could be options.

Defense Secretary Ash Carter spoke to his Turkish counterpart by phone Tuesday, offering support and discussing the importance of Incirlik Air Base. US officials are indicating they are hopeful the Turks will turn the power back on in the coming days. But no firm promises have been made so officials said that planning continues.

The fact such contingency planning is underway for dealing with a NATO ally is perhaps the clearest signal to date that the US remains uncertain how much it can rely on Turkish military cooperation in the wake of the continuing aftermath of the failed coup by elements of the Turkish military.

Defense officials said the Pentagon wants to find out not just when the power will be restored at Incirlik, but also get a better idea of the way ahead for the military in Turkey and how it will operate in the wake of the coup.

The Turks still have not given a clear explanation about why they cut the power to Incirlik.

State Department spokesman Mark Toner told reporters Tuesday, however, that while "I don't have a determination one way or the other" on whether the power was deliberately being kept off, "I don't believe so, no."

Asked whether it was purely a power issue or now also a diplomatic one, Toner replied, "My understanding is that it's a power issue."

Steven Cook, an expert on Turkish politics with the Council on Foreign Relations, suggested Tuesday that the power was cut in part "to demonstrate Turks have some leverage over the US."

Already the Turks announced their minister of defense and foreign minister would not be attending an upcoming meeting in Washington on ISIS operations long in the works.

The US has conducted airstrikes from the Turkish air base since August 2015 after extensive negotiations for access to the base from the Turkish government.

The Turks and US share the runway, but US personnel operate generally from one portion of the base. US officials are making it clear they do not want to have to pack up.

Cook of the Council on Foreign Relations indicated that Ankara could be signaling that -- after a year of negotiations between the US and the government -- "what the Turks gave, they can take away."

Given all the US effort to be able to use the base for attacks on ISIS, he doesn't believe the US military "is going to get out and go on its own volition," as much as it is frustrated, because of the difficulty and expense of conducting anti-ISIS missions out of UAE, Qatar or an aircraft carrier.

He added, "There will probably be some change but it will probably come from the Turks."

US defense officials said that so far they see no evidence the Turks are using the power supply cutoff as leverage to try to get the US to extradite Fetullah Gulen, a cleric who lives in exile in Pennsylvania and whom the Turks claim was behind the coup. US defense officials said that it a diplomatic and legal matter to be separately resolved.

White House spokesman Josh Earnest acknowledged yesterday that the US has received electronic documents relating to him from Turkey. The US said it needed to have evidence of Gulen's participation in the coup before moving ahead with extradition.

President Barack Obama discussed Gulen with Turkish President Recep Tayyip Erdogan in a call Tuesday, according to Earnest.

A statement describing the conversation put out after the call by the White House made no mention of Incirlik, however.

The Pentagon's primary concern is for Turkey to re-establish a senior officer corps that is a reliable partner that the US military can interact with and count on.

Officials refused to comment on whether any planning is underway to also move the nuclear weapons stockpile at Incirlik, should the need arise.