ISTANBUL - With the United States mulling sanctions over Turkey's recent procurement of Russian missiles, Ankara is warning that America's use of a critical military air base could be at risk.

The U.S.'s decades-long use of Incirlik Air Base is seen as not only of vital military importance, but underscores the strategic relationship between the two NATO allies.

"We are currently running the process [of retaliatory measures], whether it's Incirlik, Kurecik [U.S. radar base in Turkey] or other issues," Turkish Foreign Minister Mevlut Cavusoglu warned Washington this week, in the latest ratcheting up of bilateral tensions.

"If America has very negative steps toward us," he added, "if there are sanctions or further steps, we will have answers to America."

Last month, Turkish President Recep Tayyip Erdogan reportedly made a similar threat over Incirlik's use, in his meeting with President Donald Trump in June on the sidelines of the G-20 economic summit in Osaka, Japan.

FILE - President Donald Trump, right, shakes hands with Turkish President Recep Tayyip Erdogan during a meeting on the sidelines of the G-20 summit in Osaka, Japan, June 29, 2019.

This month's delivery of Russia's S-400 missile system to Turkey violates the U.S.'s Countering America's Adversaries Through Sanctions Act, or CAATSA, which prohibits major purchases of Russian military equipment. Under CAATSA, Ankara could face significant financial and economic sanctions. The S-400 purchase has already resulted in Turkey's exclusion from buying America's latest F-35 fighter jet. U.S. officials believe Turkey's decision to use the Russian advanced radar technology could compromise NATO's military systems in the country. The F-35 is NATO's newest stealth fighter jet.

Analysts say Ankara has always viewed America's use of Incirlik as significant leverage at times of bilateral tension. As far back as the Cold War, the vast air base located close to the Syrian frontier has been vital to U.S. strategic interests. According to nuclear watchdogs, as many as 50 nuclear weapons are stored at Incirlik.

American forces continue to use Incirlik in the war against the Islamic State terror group in both Iraq and Syria, as well as being an important logistics hub. But analysts say the importance of Incirlik extends far beyond military interests.

FILE - First parts of a Russian S-400 missile defense system are unloaded from a Russian plane near Ankara, Turkey, July 12, 2019.

"The military component of Turkey's anchorage in the West is very significant. In this regard, Incirlik has been one of the backbones of this relationship with the West," said Galip Dalay, a visiting scholar at Department of Politics and International Relations at Oxford University. Dalay is also a research director of the Istanbul-based Middle East research group Al Sharq Forum.

"If the Incirlik use is terminated, that would definitely create further discussion, over the future of Turkey's U.S. relationship and Turkey's place in NATO in general," he added. "In this regard, the symbolism of Incirlik is far more important than its military utility, although this is very important in terms of the war on terror in Syria and Iraq."

U.S. sanctions

Ankara in the past has threatened Washington over Incirlik's use. Onerous restrictions on its use by American planes have occasionally been imposed, but the base has remained open even at times of high diplomatic tension. Ankara again is looking to a diplomatic resolution.

"Ankara's calculation was built on the fact that all our strategy is built around the authority of Donald Trump, and until now it has worked," said former senior Turkish diplomat Aydin Selcen, referring to the U.S. president. "Trump can delay CAATSA sanctions; this is what the calculation is by Ankara."

Trump has voiced his opposition to imposing sanctions on Turkey. The U.S. president has acknowledged he has developed a good working relationship with Erdogan; however, Trump is facing growing pressure from Congress with bipartisan support for the imposition of sanctions on Turkey.

If Washington were to follow through on sanctions, then America's presence at Incirlik could be at risk. "One thing right now we've learned that most of the things we thought were bluffs turned out to be the case," said Dalay.

Incirlik Air Base, Turkey

"The purchase of S-400 — many analysts in [Washington] D.C. thought this was just a negotiating tactic, but it became clear Turkey meant it. I think many things are possible. In this regard, Incirlik [closure] is a possibility if we witness further escalation if the U.S. imposes hard CAATSA sanctions. Anything is possible."

Alternative bases

The Pentagon, however, could be taking steps to end its dependence on Incirlik, given that Turkish-U.S. relations have been strained for many years, going back to the U.S.-led invasion of Iraq. The U.S. recently spent $150 million, enhancing the Muwaffaq Salti Air Base in Jordan. Such measures could be a sign of a broader strategic shift.

"From what I understand, they [U.S.] are investing heavily in other bases," said international relations expert Soli Ozel of Istanbul's Kadir Has University. "And maybe no single base can fulfill the function of Incirlik, but maybe a combination of bases can substitute for Incirlik."

"Since 1992, Turkey created a lot of resistance over the use of Incirlik," he added. "At some point, the Americans may have started to seek alternatives and maybe now are close to finding alternatives. Which means Incirlik will no longer be the trump card that it used to be for Ankara."

Some observers have described Incirlik as the glue that helps bind the two allies together.

"If the Americans pull out of Incirlik, then the Americans are sincere to create trouble for Turkey," said international relations professor Huseyin Bagci of Ankara's Middle East Technical University. "Then I would really worry for the future of this relationship."