Donald Trump has threatened to "totally destroy and obliterate" Turkey's economy if it doesn't watch over captured Islamic State fighters detained in Syria following his decision to pull US forces out of the region.

In a tweet on Monday, the US President wrote: "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".

Mr Trump defended his sudden decision to pull back US troops from northeastern Syria, clearing the way for an expected Turkish military invasion against Kurdish forces who fought alongside American forces to defeat the Islamic State. Turkey views the Kurds as terrorists and a threat to the country.

Mr Trump's surprise move - described by the main Kurdish-led group as a "stab in the back" - goes against the advice of senior officials in the Pentagon and state department, reports news.com.au.

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Democrats and Republicans warn that a Turkish attack could lead to a massacre of the Kurds, who are holding thousands of captured IS fighters and their families.

Speaking to reporters, a senior state department official said the number of personnel being pulled back from the border area was "very small", and that they had been moved a "very short distance".

The Defence Department made it clear in a statement Tuesday that the Pentagon does not endorse the Turkish operation in Northern Syria and the US will not be involved in the operation.

Senator Lindsey Graham, a top Republican ally of Mr Trump, is threatening legislation to impose economic sanctions on Turkey if it invades Syria. Mr Graham has Democratic allies and warned that any congressional vote would be overwhelming.

Mr Trump's announcement immediately drew pushback from Republican politicians. Graham said Monday that Mr Trump's moves are a "disaster in the making" that would empower IS and Syria. He said he's already spoken to Maryland Democrat Chris Van Hollen about drafting the sanctions legislation. Graham said on Twitter that "sanctions against Turkey — if necessary — would be veto-proof." Utah Republican Sen. Mitt Romney also weighed in, saying Trump's "decision to abandon our Kurd allies in the face of an assault by Turkey is a betrayal."

Mr Trump is defending his decision to pull back US troops from northern Syria, clearing the way for an expected Turkish assault and essentially abandoning Kurdish fighters who fought alongside American forces to defeat the Islamic State.

The move drew immediate outrage from some of the Republican president's closest allies in Congress, with Republican Sen. Lindsey Graham calling the decision "a disaster."

"I hope President Trump will reassess and take sound military advice," he wrote on Twitter. "We have sent the most dangerous signal possible - America is an unreliable ally."

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Syria's Kurds accuse the US of turning its back on its allies and risking gains made in the fight against the Islamic State group.

Mr Trump defended the move in a series of tweets, acknowledging that "The Kurds fought with us" but claiming they "were paid massive amounts of money and equipment to do so."

Last year, the US raised tariffs on some Turkish products and imposed sanctions on top officials as relations between the two NATO countries worsened over a number of issues.

Turkish President Recep Tayyip Erdogan has said his aim was to combat Kurdish fighters in the border area and set up a "safe zone" for up to two million of the more than 3.6 million Syrian refugees currently living in Turkey.

In a statement, Pentagon spokesman Jonathan Hoffman said "the Department of Defence made clear to Turkey - as did the president - that we do not endorse a Turkish operation in Northern Syria".

Earlier, Mr Trump said it was time "to get out of these ridiculous Endless Wars, many of them tribal" and that "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 "neighbourhood." They all hate ISIS, have been enemies for years.

"We are 7000 miles away and will crush ISIS again if they come anywhere near us!"