Amid escalating threats to boot Turkey from its stealth fighter jet program over its purchase of Russia's S-400 air defense systems, Washington will reportedly stop accepting Turkish pilots for F-35 training.

The US has decided to stop accepting new pilots as part of the training program with Turkey, Reuters reports, citing officials. Currently, four pilots and 47 personnel are undergoing training at Luke Air Force Base in Arizona, where they are taught how to maintain and steer the fifth-generation jets.

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It is understood that the Trump administration has not yet made up its mind whether to limit the training program to the Turkish personal already in the US, or to put in on hold altogether. It was reported last week that the US was "seriously considering" suspending all training for the Turkish servicemen as Ankara sticks to its $2.5-billion deal with Moscow, paying little heed to Washington's ultimatums.

Acting US Defense Secretary Patrick Shanahan said last Friday that the US would prefer to settle the differences between the "strategic partners" through negotiations, rather than just send its pilots back in a tit-for-tat response.

"I don't want to just call him [Turkish Defense Minister Hulusi Akar] and say, ask your people to come home, that wouldn't be a respectful way between two strategic partners," he said.

Turkey offered to create a working group with the US to try to reach a compromise, but Washington has made it clear it wants Ankara to buy American Patriot systems instead of Russian systems. Turkey has insisted it's choosing what is better for its national defense, and the US' 'consolation' offer is no match for the Russian one.

READ MORE: 'None of our business': Moscow won’t interfere in Turkey-US negotiations over S-400 deal

Ankara has said time and again that the S-400 purchase is a done deal, and has not relented in the face of American threats to exclude it from the F-35 program, which has already cost Turkey over $1 billion. The deal between Russia and Turkey was struck in 2017, and the first battery may be delivered as early as this summer.

Washington argues that the S-400 systems pose a security threat to NATO infrastructure and its interoperability with Turkey. Moreover, they will supposedly compromise the security of the F-35s should Turkey get its hands on the jets.

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