Turkey could face expulsion from the F-35 program if it goes ahead with the purchase of Russian-made S-400 air defense systems, a Pentagon report said.

Citing the S-400 deal, the U.S. Congress quickly approved an amendment to the National Defense Authorization Act (NDAA) that prohibited F-35 sales to Turkey until the Pentagon issued a report on Turkish-American relations in 90 days.

In a two-page summary of the report seen by Anadolu Agency, the Pentagon stressed Turkey's "unique geostrategic position on NATO's southeastern flank" which is why the U.S. and Turkey share significant regional interests.

The report highlighted Turkey's credible plan to make its annual defense spending amount to 2 percent of the GDP, U.S. President Donald Trump's key demand from the NATO allies.

"Despite bilateral tensions, Turkey remains a productive military partner in many areas," the Pentagon said, calling Ankara "a critical NATO ally."

However, the report also said the Trump administration "will reassess Turkey's continued participation as one of the eight partner nations should they continue their purchase of the S-400."

Furthermore, the acquisition of the S-400 could trigger U.S. sanctions and could affect Turkey's purchase of other weapons, it added.

In a bid to have Turkey walk away from the deal with the Russians, Washington "has developed an alternative package to provide Turkey with a strong, capable" missile defense system, which would be compatible with NATO equipment.

According to the Pentagon, it is "essential to provide a real alternative that would encourage Turkey to walk away from a damaging S-400 acquisition."

Turkey is an important partner in the international F-35 program and is the sole source in the world for some of the warplane's parts.

"Turkish industry manufactures aircraft parts for all F-35 variants and customers," the Pentagon said.

Turkey has been in the F-35 program since 1999, and the Turkish defense industry has taken an active role in the production of aircraft and invested $1.25 billion in the aircraft's development.

The country plans to purchase 100 F-35 fighter jets in the coming years. Out of 100 aircraft, 30 have been approved. The country took delivery of its first F-35 fighter jet at a ceremony in Fort Worth, Texas, on June 21. Two more F-35s are expected to be delivered by March 2019.

In December 2017, Ankara agreed to buy two Russian-made S-400s in a deal worth over $2 billion. Turkish officials have repeatedly said that the purchase of the S-400 systems was made to fulfill the country's security needs. With the purchase of the S-400s, Ankara aims to boost its defense capabilities amid threats from PKK and Daesh terrorists at home and conflicts across its borders in Syria and Iraq.

Ankara has been disappointed with its NATO allies for their lack of cooperation in meeting its defense needs. On Oct. 25, National Defense Minister Hulusi Akar said that Turkey would begin the installment of S-400 anti-aircraft missile systems in October 2019.

Turkish officials have repeatedly stressed the purchase of the S-400s was a done deal and could not be canceled.