- The US troops have served as a buffer between the Turkish military and the Kurdish-led Syrian Democratic Forces
- The Kurds lead the Syrian Democratic Forces, who were essential in the US-led coalition that defeated the Islamic State.
- Turkey regards Kurdish forces within the SDF as a terror threat because it maintains ties to Kurdish militants inside Turkey, and has vowed to crush them.
US President Donald Trump warned Turkey on Monday against going too far in Syria, one day after giving Ankara a green light to invade its southern neighbour 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 between the Turkish military and the Kurdish-led Syrian Democratic Forces, after Trump's surprise announcement in advance of an expected operation ordered by Ankara.
In a day of foreign policy turmoil, Trump came under heavy criticism from top Republicans for abandoning the Kurds, who were crucial in the years-long campaign to defeat Islamic State.
Trump appeared to then backpedal, warning Ankara he would "obliterate" the country's economy if they 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.
However Trump also said 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."
A senior administration official denied Trump had given a "green light" to an invasion, which raised the threat that thousands of Islamic State fighters that the Kurds hold prisoner could be freed amid a fresh conflict.
"It appears the Turks are intent on some sort of military operation," the official said on grounds of anonymity, adding: "There will be no US armed forces involvement."
Even Trump's allies accused him of turning his back on the Kurds, who say they lost some 11,000 fighters taking the lead in the battle against the Islamic State.
"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 criticised Trump.
"And it would increase the risk that ISIS and other terrorist groups regroup," he said.
'PREPARE FOR WAR'
Turkish President Recep Tayyip Erdogan said Monday that the operation 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.