Trump Threatens Turkey after Greenlighting Syria Invasion

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. 

In a day of foreign policy turmoil, Trump's surprise announcement on Monday drew heavy criticism from top Republicans that he was abandoning the Kurds, who were crucial in the years-long campaign to defeat Islamic State.

Trump appeared to then backpedal, warning Turkey he would "obliterate" the country's economy if it went too far -- without explaining what that meant.

"I have told Turkey that if they do anything outside of what we would think is humane... they could suffer the wrath of an extremely decimated economy," he said.

Ankara brushed aside the threat, with Vice-President Fuat Oktay saying: "Turkey is not a country that will act according to threats."

Iran, a key backer of the Syrian government, said on Tuesday it opposed any military action by Turkey. 

In a call to Ankara, Foreign Minister Mohammad Javad Zarif "urged respect for Syria's territorial integrity".

Turkey has always pushed hard against Washington's support for Kurdish forces in Syria, fearing it would strengthen Kurdish separatists in its own territory. 

Its defense ministry tweeted that preparations for an offensive in northern Syria have been "completed".

A senior US official denied Trump had given a "green light" to a Turkish invasion, which has also raised concerns over the fate of thousands of Islamic State prisoners held in Kurdish detention centres. 

"It appears the Turks are intent on some sort of military operation," the official said on condition of anonymity, adding: "There will be no US armed forces involvement."

Trump emphasised he wanted to end the US military presence in the region.

"We want to bring our troops back home from these endless wars and we're like a police force. We're not fighting. We're policing."

- US allies abandoned? -

Even Trump's allies accused him of turning his back on the Kurds, who lead the Syrian Democratic Forces (SDF) and say they lost some 11,000 fighters as the main frontline force against the Islamic State group.

"A precipitous withdrawal of US forces from Syria would only benefit Russia, Iran, and the Assad regime," said Senate Majority leader Mitch McConnell, one of the senior Republicans who criticized Trump.

"And it would increase the risk that ISIS and other terrorist groups regroup," he said.

Turkish President Recep Tayyip Erdogan said Monday that the operation into Syria could "come any night without warning," while Kurds in the area girded for fighting.

"The prudent should prepare for war," said Mustefa Bozan, a 79-year-old shopkeeper in Ras al-Ain, where a contingent of US Special Forces had been until early Monday.

Ankara says it wants to establish a "safe zone" on the Syrian side of the border where it could send back some of the 3.6 million refugees from the eight-year civil war.

Kurds argue that Ankara's goal is to dilute their dominance in the region with an influx of mostly Sunni Arab refugees now living in southwestern Turkey.

- Pentagon warns of destabilization -

Trump's announcement appeared to have caught many US officials by surprise, and they spent Monday seeking to discourage Ankara from acting.

Pentagon spokesman Jonathan Hoffman warned Turkey of destabilizing blowback to the region if it invades.

"The Department of Defense made clear to Turkey -- as did the president -- that we do not endorse a Turkish operation in Northern Syria," he said.

SDF spokesman Mustefa Bali said Washington's decision "is about to ruin the trust and cooperation between the SDF and US."

"As the Syrian Democratic Forces, we are determined to defend our land at all costs," the group said.

A Turkish attack raised the specter of some 10,000 Islamic State fighters currently held by the SDF being let loose if SDF soldiers have to battle Turkish troops.

Around 2,000 of them are IS "foreign fighters", and Trump assailed US allies in Europe for not taking back their nationals.

If they escape or are released, the could reconstitute the Islamic State, less than one year after it was defeated and its "caliphate" disintegrated.

Trump declared that it would be the responsibility of Turkey and other countries to deal with the IS prisoners.

"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,'" he said, using another acronym for the jihadists.