Turkey’s President Recep Tayyip Erdogan threw away a letter from President Donald Trump urging negotiations on Ankara’s invasion of north-east Syria.

The letter, dated October 9 – the day Turkish troops entered Syria and one day after Mr Trump said he was pulling American soldiers back – urged the Turkish leader “don’t be a tough guy. Don’t be a fool.”

However, reports from the BBC say that “President Erdogan received the letter, thoroughly rejected it and put it in the bin."

In Mr Trump’s letter, obtained by US journalists on Wednesday – a week after it was sent – the president appealed to Mr Erdogan to halt the offensive and warned of dire consequences if he didn’t.

"Let's work out a good deal! 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 - and I will," Mr Trump wrote.

"History will look upon you favourably if you get this done the right and humane way. It will look upon you forever as the devil if good things don't happen."

An October 9 letter from US President Donald Trump to Turkey's President Tayyip Erdogan warning against a military offensive in Syria. The White House, HO

Russia, a key backer of the Syrian government and a country with close ties to Turkey, questioned the tone of Mr Trump’s letter. The Kremlin called it a highly unusual correspondence between heads of state.

"You don't often encounter such language in correspondence between heads of state. It's a highly unusual letter," Kremlin spokesman Dmitry Peskov told reporters.

The reports of the letter came as Vice President Mike Pence and Secretary of State Mike Pompeo set off for talks with Mr Erdogan in which they were expected to reiterate Mr Trump’s call for a halt in fighting.

However, Mr Erdogan has said that no force can halt the offensive before a 32-kilometre buffer zone has been established inside Syria. Turkey has warned that they face an existential threat from the YPG, a Kurdish militia force that makes up the US-backed Syrian Democratic forces but are seen by Ankara as an offshoot of the banned PKK militia with whom they have fought a decadeslong insurgency in Turkey.

Turkey also says it is preparing to issue its own sanctions against the US in response to measures taken by Washington in response to the offensive.

While warning Turkey over the assault, Mr Trump appears to be shifting his position on Kurdish forces – having previously praised them for leading the internationally-backed fight against ISIS in Syria.

Mr Trump suggested on Wednesday that the YPG was a greater terror threat than ISIS and welcomed the efforts of Russia and the government of Bashar Al Assad to fill the void left after he ordered the removal of nearly all US troops from Syria.

He also again indicated he did not want to become more involved.

“Syria may have some help with Russia, and that’s fine,” Trump said. “They’ve got a lot of sand over there. So, there’s a lot of sand that they can play with.”

He added: “Let them fight their own wars.”

The White House now appears to be perusing a split policy towards Syria – offering up severe condemnation the Turkish offensive but also embracing of Turkish talking points about the former US allies.

In response to the offensive, Kurdish-led authorities in the self-governed area of Syria have made a deal with Damascus to see the government send troops to the area to push back Turkey and its allied militias.

The move has overturned the map of Syria and see the return of the regime to large parts of northern Syria, eight years after they abandoned the territory at the early stages of the war.

Meanwhile, Turkey came under renewed scrutiny on Thursday with Kurdish forces accusing Ankara of using banned munitions in the fight to capture the town of Ras Al Ain – a border village they initially took before being pushed back by SDF forces.

"The Turkish aggression is using all available weapons against Ras Al Ain," Kurdish authorities said in a statement, accusing Ankara of firing napalm and white phosphorus – two internationally outlawed weapons – and harming civilians.

"We urge international organisations to send their teams to investigate some wounds sustained in attacks," Mustafa Bali, a spokesman for the SDF, said on social media. "The medical facilities in NE Syria lack expert teams," he added.

The Rojava Information Centre, a media centre in northeast Syria, shared what they said was the autopsy report of Hevrin Khalaf, a Kurdish politician reportedly killed by Turkish allied Syrian armed groups earlier this week.

The report says her wounds indicated that she was pulled by the hair, beaten with a blunt object in the head and legs before being shot in the head and body a number of times at close range.

Kurdish authorities have accused Turkey’s Syrian allies of ‘executing’ her, her driver a number of civilians as well as several captured Kurdish soldiers.

The UN has warned that Turkey may be held responsible for war crimes if investigations confirm the events surrounding the nine killings.

Mr Trump’s letter came out around the same time as a meeting between the president and his senior officials with Democratic and Republican politicians, including house leader Nancy Pelosi and Senate leader Chuck Schumer.

The meeting descended into chaos with Ms Pelosi walking out and accusing the president of having a “meltdown”. The president made the same accusation against Ms Pelosi, sharing a photo of what he described as the exchange. Ms Pelosi later tweeted the same image of her standing across from the president with her finger outstretched in accusation and made it the cover photo of her Twitter account.

Republican leaders said Ms Pelosi's behaviour was "unbecoming", and criticised her for "storming out".

However, the issue of a US withdrawal from Syria has led to rare bipartisan action with 129 members of the Republican Party joining Democrats in the House to denounce the move in a vote on Wednesday that passed 354-60.

The joint resolution called on President Erdogan to immediately cease military operations.

Senate Majority Leader Mitch McConnell publicly broke with Mr Trump to call the US relationship with the Kurds “a great alliance.”

“I’m sorry that we are where we are. I hope the vice president and the secretary of state can somehow repair the damage,” Mr McConnell said Wednesday.

Turkish President Tayyip Erdogan meets US Vice President Mike Pence at the Presidential Palace in Ankara. Reuters

Even among top administration officials, there were concerns that the trip by Mr Pence and Mr Pompeo lacked achievable goals and had been undermined by Mr Trump even before it began. While Mr Erdogan faces global condemnation for the invasion, he also sees renewed nationalistic fervour at home and support from near all the local political spectrum. Any pathway to de-escalation would likely need to delicately avoid embarrassing Mr Erdogan domestically, officials talking off the record told the Associated Press.

The withdrawal is the worst decision of the Trump presidency, said Senator Lindsey Graham, who meets often with the president and is one of his strongest and most important supporters in Congress.

“To those who think the Mideast doesn’t matter to America, remember 9/11 – we had that same attitude on 9/10/2001,” Mr Graham said.