US President Donald Trump last week warned Turkish counterpart Recep Tayyip Erdogan in a letter that he would soon have to impose sanctions on Turkey over its purchase of Russian-made S-400 missile defence systems if Ankara did not accept his proposed terms, Middle East Eye has learned.

According to MEE's sources, Trump also said that Turkey could be re-admitted into a partner programme for the US's next-generation F-35 fighter jet if it agreed not to activate the S-400 systems and committed to not purchasing Russian weapons systems in the future.

Erdogan is scheduled to meet Trump on Wednesday in Washington with the issue of Turkey's S-400 deal expected to be the main topic of conversation, even with the two NATO allies at odds over a range of issues including Ankara's incursion into northern Syria to confront Kurdish forces formerly backed by the US.

The letter, according to two Washington sources who are familiar with the matter, aimed to lay the groundwork for the discussions between the two leaders this week.

Trump's last correspondence with Erdogan, sent last month and threatening him with heavy sanctions over Turkey's operation in northern Syria, caused uproar at the time because of its informal style which was perceived as undiplomatic and "childish".

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"Let's work out a good deal!.... Don't be a tough guy! Don't be a fool!" Trump wrote.

Turkish officials told MEE at the time that the letter was dumped in a rubbish bin. Trump, this time, used diplomatic language that is in line with established state protocols, sources told MEE.

The White House, under pressure from the Congress, seems to have lost patience and really wants to put an end to the S-400 debacle, by either sanctioning Turkey, or using the threat of sanctions to force it to accept its terms, sources said.

Trump, in the letter that was delivered to Erdogan’s senior aide Ibrahim Kalin last Monday, offered a specific way to resolve the problem without imposing penalties: Turkey should not activate S-400 systems, allow regular visits by US officials to check the system status, and Ankara should commit itself not to make further purchases of Russian weaponry.

$100bn trade deal

If Erdogan accepts his terms, Trump also committed himself to reinstating Turkey in the F-35 programme, and drafting a bilateral trade deal worth of $100bn.

Though not mentioned in the letter, Trump has publicly said multiple times that Turkey should purchase US-made Patriot missile systems to defend its territories.

Erdogan told journalists last week that he was still interested in the Patriots.

Trump's request over the S-400s stems from the fact that his administration is mandated to sanction countries that conduct transactions with the Russian military industry, according to a law ratified by the US Congress in 2017, called CAATSA, or Countering America's Adversaries Through Sanctions Act.

Even though Turkey signed the deal with Moscow well before the purchase, US diplomats have said it is still sanctionable.

Turkey began to receive the first and second batch of the S-400 missile systems over the summer. The Pentagon suspended Turkey from F-35 programme in July, saying that the S-400s could be used to spy on the crucial technology of the jet.

If Turkey could return to the programme, it could purchase nearly 120 jets from the US. Turkey also could continue to produce parts of the jet that could bring $9bn in income to Turkish companies over the life of the programme.

Erdogan not convinced

However, Trump's letter, it seems, did not convince Erdogan, according to sources.

Erdogan, replying by letter, told Trump that Turkey would not discuss the S-400 issue with pre-conditions.

He reiterated his demand to form a joint committee to resolve the problem and continued to defend the argument that S-400s and F-35 could be compatible within Turkey's defence structure.

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The Turkish leadership continues to believe that Erdogan could convince Trump to find an alternative solution to the crisis when the two meet for lunch and an expanded bilateral meeting at the White House.

Robert O'Brien, Trump's new national security adviser, said on Sunday that the US would do its "very best" to keep Turkey within NATO.

Asked in an interview with CBS whether Trump would veto sanctions against Turkey, O’Brien said, "We have to see what happens. If Turkey won't get rid of the 400, Turkey will feel the impact of those sanctions.

"There is no place in NATO for the S-400 and for Russian purchases, and that's a message that the president will deliver to him very clearly when he is here in Washington."

When contacted by the MEE, the White House declined to comment for the story. The Turkish Presidency has declined to comment.