'Don't be a tough guy, don't be a fool!': Trump's extraordinary letter to Erdogan prompts disbelief

White House forced to deny it was not a hoax

President urged his counterpart to 'do a deal'. (AFP via Getty Images)

Donald Trump warned Turkish president Recep Tayyip Erdogan not to be "a tough guy" on the day his forces crossed the border into Syria, an extraordinary letter from the US leader has revealed.

"History will look upon you favourably if you get this done the right and humane way," the US president said in the letter released by the White House, which was later forced to confirm that it was real after disbelieving journalists assumed it must be a hoax.

"It will look upon you forever as the devil if good things don't happen," it continued.

"Don't be a tough guy. Don't be a fool! I will call you later."

Using his favourite catchphrase, he said: “Let’s work out a good deal!"

He added: "You don’t want to be responsible for slaughtering thousands of people, and I don’t want to be responsible for destroying the Turkish economy.”

Mr Trump went on to suggest that General Mazloum Kobani, commander of the Kurdish army known as the Syrian Democratic Forces (SDF), was willing to negotiate with Mr Erdogan.

Funded and trained by the US, the SDF led the battle against Isis in northern Syria.

General Mazloum was "willing to make concessions that they would never have made in the past," Mr Trump wrote, adding that he was "confidently enclosing a copy of his letter to me, just received."

Mr Trump had earlier referred to the letter during a press conference with Italian President Sergio Mattarella, while he argued that he had not given Mr Erdogan the "green light" to invade Syria.

"If anybody saw the letter, which can be released very easily if you'd like, I could certainly release it," he said. "But I wrote a letter right after that conversation – a very powerful letter. There was never given a green light."

However, the letter was dated 9 October, three days after he announced his intention to withdraw troops from Syria, following a phone call with Mr Erdogan.

The letter was initially leaked to Fox News host Trish Reagan, then confirmed by the White House as some questioned its authenticity and others suggested it was parody.

It was released after he was accused of having a “meltdown” at a White House in a meeting with House leadership, after Congress passed a bipartisan resolution rebuking him for his withdrawal of troops in northeastern Syria.

Senate Minority Leader Chuck Schumer, a New York Democrat, said the president turned the meeting into a “diatribe” while attacking House Speaker Nancy Pelosi.

Mr Trump "was insulting, particularly to the speaker," Mr Schumer said. "He called her a third-rate politician."

"This was not a dialogue. It was sort of a diatribe," he added. "Not focused on the facts, particularly the facts about how to contain Isis."

Ms Pelosi also announced the White House had cancelled a classified briefing scheduled for Thursday with House members about the latest developments in Syria.

She said scrapping the meeting prevents Congress from learning about “the dangerous situation” caused by Mr Trump’s withdrawal of troops in the region.

Ms Pelosi said she was “deeply concerned” because House members have a right to be informed about such decisions.

The White House disputed the claims, saying Mr Trump had been "measured, factual and decisive"

It added: "While democratic leadership chose to storm out and get in front of the cameras to whine, everyone else in the meeting chose to stay in the room and work on behalf of this country.”