President Trump defended his move again on Tuesday, saying the Kurds had not been abandoned, calling them “special”.
His controversial decision was widely condemned at home and abroad.
Turkey regards the Kurdish militias, which dominate the US-allied Syrian Democratic Forces (SDF), as terrorists, and the US pullout was seen as paving the way for an offensive.
Convoys of trucks with armoured personnel carriers and tanks were seen heading to the Turkish border town of Akcakale on Tuesday night. Images of buses carrying Turkish commandos were shown by state news agency Anadolu.
Trump: Kurds ‘wonderful fighters’
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 some 1,000 US troops in the country.
In a series of tweets, Mr Trump softened his tone, praising Turkey as a trade partner and Nato ally, hours after saying he would “destroy and obliterate” its economy if the country went “off limits”.