Turkey has said it is ready to launch its long-threatened operation in north-eastern Syria that could target Kurdish forces long allied to the US.
The announcement came after President Trump ordered a withdrawal of US troops from the border area, a move that was widely condemned at home and abroad.
Mr Trump defended his move again on Tuesday, saying the Kurds had not been abandoned, calling them "special".
He softened his tone after threatening Turkey if it acted "off limits".
In a series of tweets, Mr Trump praised Turkey as a trade partner and Nato ally, hours after saying he would "destroy and obliterate" its economy if the country took advantage of his surprise decision.
Mr Trump said his pullout – described as a "stab in the back" by Kurdish forces who helped defeat Islamic State (IS) in Syria – affected "only 50 soldiers" of the some 1,000 US troops in the country.
"We may be in the process of leaving Syria, but in no way have we Abandoned the Kurds, who are special people and wonderful fighters," Mr Trump said, adding that the US was helping the Kurds "financially [and with] weapons".
"Any unforced or unnecessary fighting by Turkey will be devastating to their economy and to their very fragile currency," the president said, describing the relationship between the two countries as "very good".
Turkey regards the Kurdish militias, which dominate the US-allied Syrian Democratic Forces (SDF), as terrorists, and the US withdrawal was seen as paving the way for a Turkish offensive. On Tuesday night, military vehicles and personnel were seen heading towards the border area.
Meanwhile, the White House confirmed that Turkish President Recep Tayyip Erdogan would visit the US on 13 November at the invitation of President Trump.
Turkey wants to set up a 480km (300 mile) long and 30km deep "safe zone" along its border with Syria, to resettle up to two million of the more than 3.6 million Syrian refugees currently living in Turkey.
Eyes trained on the border
BBC's Orla Guerin in Akcakale on the Turkish-Syria frontier
On both sides of the border wall there is a sense that an offensive is coming, and probably soon. There is no indication that the crescendo of international concern is altering Turkey's plans for establishing a "safe zone".
Dozens of TV cameras – local and international – are now trained on the border wall. For civilians and refugees on the other side, in north-eastern Syria, there are real fears of what a Turkish invasion could mean.
Even if it is limited in scope – and it is unclear if it will be – it could cause massive displacement. And what of the thousands of Islamic State prisoners being held in Kurdish-run detention centres?
A spokesman for the SDF told the BBC that if the attack happened, they would have to focus on defending themselves and would have to withdraw some of their forces from the jails, and from areas recently liberated from IS.
What's Turkey saying?
Turkey's defence ministry said "all preparations for the operation have been completRead More – Source