Washington (CNN) In an extraordinary letter sent to Turkish President Recep Tayyip Erdogan last week, President Donald Trump warned that he could destroy Turkey's economy if the situation in Syria is not contained and resolved in a humane way, CNN has confirmed.

The tone of the letter is consistent with Trump's unconventional approach to diplomacy, particularly when dealing with strongman leaders, as he tells Erdogan: "Let's work out a good deal" and "Don't be a fool!"

But it's striking to see a US President use such language. Trump also warned his Turkish counterpart not to "be a tough guy!"

The letter is dated October 9 -- three days after the two leaders had spoken by phone -- the same day the Turkish incursion into Syria began, a detail that raises more questions about what Trump said the Erdogan as compared to the contents of his written correspondence.

"Don't let the world down," Trump wrote, adding that the Kurds are willing to negotiate and willing to "make concessions that they never would have made in the past."

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