US President Donald Trump has warned Turkey against going too far in Syria, just a day after giving Ankara an apparent green light to invade its southern neighbor and sparking panic among Washington's Kurdish allies.

The US pulled back 50 to 100 "special operators" from Syria's northern frontier, where they have served as a buffer preventing a long-planned assault by the Turkish military against Kurdish forces in Syria. 

Turkey's preparations for an offensive in northern Syria have been "completed", the defence ministry said Tuesday, after confusing signals from the US over whether it would allow an operation.

US President Donald Trump ordered the withdrawal of US troops from Turkey's border with Syria after a phone call with his counterpart Recep Tayyip Erdogan. 

Iran's top diplomat Mohammad Javad Zarif told his Turkish counterpart Mevlut Cavusoglu in a phone call that Tehran is opposed to military action in Syria, the foreign ministry said.

A feared Turkish invasion of northeast Syria could spark a resurgence of the Islamic State group, analysts and Kurdish forces have warned, despite Ankara's pledge to prevent the jihadists' return.

The Pentagon said Monday the United States does not endorse a threatened Turkish military invasion of northern Syria, and cautioned that such a move risked destabilizing the region.

US President Donald Trump said Monday that he will "obliterate" Turkey's economy if Ankara does anything that in his "great and unmatched wisdom" he considers to be "off limits" in Syria.

Trump's extraordinary Twitter warning against NATO ally Turkey came just after the White House announced a U.S. military drawdown in Turkish-Syrian border areas.

U.S. forces in Syria started pulling back Monday from Turkish border areas, opening the way for Ankara's threatened military invasion and heightening fears of a jihadist resurgence.

Iraq's military admitted for the first time Monday it had used "excessive force" in nearly a week of deadly protests, as paramilitary units said they were ready to back the government.

The European Union on Monday warned that a threatened Turkish offensive against a Kurdish militia in northeast Syria would harm civilians and cause a "massive displacement" of people.

Turkish President Recep Tayyip Erdogan said Monday that his army was ready to launch operations against Kurdish militants in Syria at any moment following the US announcement that it would not stand in the way.