Turkey has boosted its forces on the Syrian border ahead of a long-threatened incursion that could target Kurdish-led forces allied to the US.
Troops would cross into Syria “shortly”, a presidential aide said.
Turkey wants to create a “safe zone” cleared of a Kurdish militia that will also house some of Turkey’s 3.6 million Syrian refugees.
Donald Trump has withdrawn US troops in the area in a controversial decision widely condemned at home and abroad.
He again defended his move on Tuesday, saying the Kurds had not been abandoned, calling them “special”.
The Kurds were key US allies in defeating the jihadist group Islamic State IS in Syria.
But Turkey regards the Kurdish militia that dominates the US-allied Syrian Democratic Forces SDF as a terrorist group. The US pullout was seen as paving the way for an offensive.
Reactions so far
The SDF said the border areas were “on the edge of a possible humanitarian catastrophe”.
“The attack will spill the blood of thousands of innocent civilians,” the SDF’s general command said.
The Kurdish-led administration in the region said in a statement: “We announce three days of general mobilisation in northern and eastern Syria,” calling on civilians to “head to the border with Turkey to fulfill their duty”.
Russian Foreign Minister Sergei Lavrov said Washington was sending mixed signals and Kurds feared the confusion could “ignite the whole region”.
Russia’s security council said it was “important for everyone to avoid any action that could create obstacles to a peaceful settlement in Syria”.
President Donald Trump’s stance
He said his pullout which was described as a “stab in the back” by the SDF, affected “only 50 soldiers” of some 1,000 US troops in the country.
In a series of tweets, Mr Trump also praised Turkey as a trade partner and NATO ally, hours after saying he would “destroy and obliterate” its economy if the country went “off limits” in its incursion.
“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 as he described the relationship between the two countries as “very good”.
In a statement, the Pentagon said “unfortunately, Turkey has chosen to act unilaterally” and that the US personnel were removed “to ensure their safety” but that they were not being sent out of Syria.
The White House also confirmed that Turkish President Recep Tayyip Erdogan would visit the US on November 13 at the invitation of President Trump.