Video report by ITV News Correspondent Emma Murphy

President Donald Trump has defended his sudden decision to pull back US troops from northern Syria in a series of tweets on Monday after facing criticism from some of his closest allies.

He also threatened to destroy the Turkish economy if it acted in a way he considered - in what he said was his "great and unmatched wisdom" - "off-limits".

The move to withdraw troops from the region is a major shift in US policy and essentially abandons Kurdish fighters - who fought alongside American forces in the years-long battle to defeat so-called Islamic State (IS) - to a likely Turkish assault.

Earlier on Monday, Turkish President Recep Tayyip Erdogan confirmed US troops had started withdrawing from positions in the region, while Kurdish Hawar news agency and the UK-based Syrian Observatory for Human Rights also said American troops were evacuating positions near the towns of Ras al-Ayn and Tal Abyad.

US launches an attack on the Syrian capital Damascus in 2018. Credit: AP

Acknowledging that "the Kurds fought with us," but claiming they "were paid massive amounts of money and equipment to do so," Mr Trump said it was time for the US to "get out of these ridiculous Endless Wars."

Syria's Kurds accused the US of turning its back on its allies and risking gains made in the fight against the Islamic State group while Republican Sen. Lindsey Graham, one of Mr Trump's closest allies in Congress, called the decision "a disaster".

But the president remained defiant in the face of the criticism, taking to Twitter to defend his decision.

In one series of tweets on Monday afternoon, Mr Trump claimed he would be watching Turkey's response closely.

He wrote: "As I have stated strongly before, and just to reiterate, if Turkey does anything that I, in my great and unmatched wisdom, consider to be off limits, I will totally destroy and obliterate the Economy of Turkey (I’ve done before!). They must, with Europe and others, watch over."

Turkish and American armoured vehicles patrol as they conduct joint ground patrols in the so-called ‘safe zone’ on the Syrian side of the border with Turkey Credit: AP

Earlier, he had written: "I held off this fight for almost 3 years, but it is time for us to get out of these ridiculous Endless Wars, many of them tribal, and bring our soldiers home," he wrote, adding that, "WE WILL FIGHT WHERE IT IS TO OUR BENEFIT, AND ONLY FIGHT TO WIN," he wrote.

He said "Turkey, Europe, Syria, Iran, Iraq, Russia and the Kurds will now have to "figure the situation out, and what they want to do with the captured ISIS fighters in their “neighborhood.”

The president warned of retribution in response to any future attacks, writing: "We are 7000 miles away and will crush ISIS again if they come anywhere near us!"

US-backed Syrian Democratic Forces (SDF) at a ceremony to mark their defeat of Islamic State militants in Baghouz, at al-Omar Oil Field base, Syria, March 23, 2019. Credit: AP

Turkey has threatened for months to launch a military operation to drive away Syrian Kurdish fighters from a border region east of the Euphrates River.

Mr Erdogan said Turkey is determined to halt what it perceives as threats from the Syrian Kurdish fighters.

In the US, Republicans and Democrats have warned that allowing the Turkish attack could lead to a massacre of the Kurds and send a troubling message to American allies across the globe.

The SDF said: "The American forces did not abide by their commitments and withdrew their forces along the border with Turkey.

"Turkey now is preparing to invade northern and eastern parts of Syria."

Turkish President Recep Tayyip Erdogan has threatened to launch a military assault against Kurdish forces across the border. Credit: Presidential Press Pool/AP

In March, the group captured the last sliver of land held by the extremists, marking the end of the so-called caliphate that was declared by IS's leader Abu Bakr al-Baghdadi in 2014.

The SDF said: "We will not hesitate for a moment in defending our people" against Turkish troops, adding that it has lost 11,000 fighters in the war against IS in Syria.

A Turkish attack would lead to a resurgence of IS, it said. IS sleeper cells are already plotting to break free some 12,000 militants detained by Syrian Kurdish fighters in north-eastern Syria in a "threat to local and international security".

SDF fighters have been integral in the defeat of so-called Islamic State. Credit: AP

The Kurdish fighters also control the al-Hol camp, home to more than 70,000 mostly wives and children of IS fighters.

Turkish foreign minister Mevlut Cavusoglu tweeted that since the beginning of the crisis in Syria, "we have supported the territorial integrity of this country, and we will continue to support it".

He added that Ankara is determined to ensure the survival and security of Turkey "by clearing the region from terrorists. We will contribute to peace, peace and stability in Syria".