Trump's latest tweets followed a storm of criticism from his own Republican party.

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

The Pentagon pulled back about two dozen of its forces from Syria's northern frontier, where they serve as a buffer between the Turkish military and the Kurdish-led Syrian Democratic Forces, after Trump's surprise announcement late Sunday that US forces "will no longer be in the immediate area."

But amid an outcry from the region and strong opposition at home from both Democratic and Republican lawmakers, the US leader appeared to reverse himself, though without drawing any specific red lines that might protect Kurdish allies.

"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 tweeted.

Other US officials, apparently surprised by Trump's Sunday announcement, stressed that Washington will not actively support the long-threated Turkish action, warning of destabilizing blowback to the region.

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

A senior State Department official said the United States would not "act militarily" to stop the Turks.

However, the official added, "We think this operation is a very bad idea. We do not think this operation will provide more security."

Invasion could 'come any night'

The region has been on edge for weeks over expected Turkish military operations across the border, which President Recep Tayyip Erdogan warned Monday could "come any night without warning."

It would threaten the Kurds who dominate the SDF militia, a crucial partner in the US-led coalition that defeated the Islamic State and currently controls much of northeastern Syria.

Turkey regards the SDF as a terror threat because it maintains ties to Kurdish militants inside Turkey, and it has vowed to crush them.

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.

IS fighters could be freed

Trump said via Twitter Monday that he wanted to extricate the United States from ""endless and ridiculous wars."

But a Turkish attack could force the SDF to free some 10,000 Islamic State fighters captured in battle over the past two years, potentially allowing them to reconstitute the violent jihadist group.

Trump declared that it would be the responsiblity of Turkey and other countries to deal with the ISIS 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.'"

Fearing yet another chapter of bloodshed and mass displacement, the United Nations said it was "preparing for the worst".

The European Union warned that civilians would once again bear the brunt of a military assault.

"We call on Turkey to avoid any initiative that would run counter to the interests of the global coalition" fighting IS in Syria, the French foreign ministry said in a statement.

Minimal US pullback

Washington officials minimized the pullback of troops that Trump had announced late Sunday.

The State Department official indicated that the number pulled back -- out of as many as a thousand estimated in place along the border -- was "two very small detachments" numbering about 25 people, who would move only a "very short distance."

"We don't want those troops to be there and thus send a signal that they are supporting the operation," the officials said.

"Beyond that, there's no change to our military posture in the northeast."

Trenches and tunnels

The Syrian Observatory for Human Rights war monitor confirmed Monday that US forces had departed key positions in Ras al-Ain and Tal Abyad, where Kurdish forces have dug trenches and tunnels to defend against attack.

A US base in Ras al-Ain was empty of personnel on Monday, said an AFP photographer, who had seen troops there the night before.

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

The group noted it had already lost some 11,000 fighters in the fight against Islamic State.

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

'Prepare for war'

In Ras al-Ain on Sunday, local armed forces allied with the Kurdish administration stood behind dirt berms, monitoring the frontier.

They had covered streets in the town with large metal canopies to block the view of Turkish drones, sources there told AFP.

"The prudent should prepare for war," said Mustefa Bozan, a 79-year-old shopkeeper.

"The fate of the region will be the same as that of Afrin," 38-year-old Issam Daoud said, referring to a former Kurdish enclave captured by Turkish troops and Syrian rebels last year.