Turkey could be preparing to launch military actions against Kurdish forces in northeastern Syria despite strong warnings from United States President Donald Trump who threatened to destroy Turkey's economy if the country does anything that he does not like.

Trump warned Ankara not to use the US troop withdrawal, announced on Monday, to take any action that he described as "off-limits" such as a Turkish attack on the Kurds, who have fought alongside the US military for years.

The abrupt US foreign policy change that will, in effect, abandon the Kurds means Turkey would take custody of captured Islamic State fighters, the White House said. The policy swerve comes after a phone conversation between Trump and Turkish President Recep Tayyip Erdogan on Sunday.

Erdogan now seems determined to carry out an offensive against the Kurdish-dominated Syrian Democratic Forces, which he views as a terrorist organization.

The Turkish leader says the creation of what he describes as a "safe zone" along the border could be used to settle up to 2 million Syrian refugees currently living in Turkey.

Kurdish forces have, up until now, spearheaded the campaign against Islamic State in the region.

Trump's critics say any Turkish offensive would add a dangerous new dynamic to the conflict in Syria, which could trigger unintended consequences.

Allowing a Turkish military offensive in the region is a move that even strong backers of the president have blasted as "shortsighted and irresponsible".

One of Trump's most vocal supporters, senator Lindsey Graham, led the backlash.

"This impulsive decision by the president has undone all the gains we've made, thrown the region into further chaos. Iran is licking their chops. And, if I'm an Isis fighter I've got a second lease on life. So to those who think Isis has been defeated you will soon see," said Graham in an interview on Fox and Friends.

Other prominent supporters of the president, including senator Marco Rubio and former UN ambassador Nikki Haley, also denounced Trump's decision.

Ruben Gallego, an Arizona Democrat who served as a Marine in the Iraq war derided the president's decision.

"Allowing Turkey to move into Northern Syria is one of the most destabilizing moves we can do in the Middle East," Gallego wrote on Twitter on Sunday evening.

"The Kurds will never trust America again. They will look for new alliances or independence to protect themselves."

In apparent response to that criticism, Trump pledged to destroy Turkey's economy if it steps over the line.

"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!)," Trump wrote on Twitter on Monday morning.

He continued: "They must, with Europe and others, watch over the captured ISIS fighters and families. The US has done far more than anyone could have ever expected, including the capture of 100% of the ISIS Caliphate. It is time now for others in the region, some of great wealth, to protect their own territory. THE USA IS GREAT!"

The Syrian Defence Forces — a largely Kurdish militia — released a statement claiming it had done its part in working to reduce tensions with Turkey, but that the US had not held up its end of the bargain.