Secretary of State Rex Tillerson said President Donald Trump did not want to “make the same mistakes” that were made in 2011, when US forces left Iraq.
The US has about 2,000 troops in Syria.
Mr Tillerson denied the US was training a Kurdish-led border force, but Turkey accused it of sending mixed signals.
“US officials have made statements that refute one another,” said Turkish Prime Minister Binali Yildirim, whose country fiercely opposes such a move.
“One day it was said that a new border force had been set up, another day they said they are setting up a unit with local forces in order to maintain security in the region after eliminating Daesh [Islamic State group, or IS] in the region. These are all confusing statements.”
The US secretary of state said officials had “misspoke[n]” when they said the US was planning to set up a 30,000 strong “border security force” in northern Syria underpinned by the allied Kurdish-led Syrian Democratic Forces (SDF) militia.
Mr Tillerson said the US was not creating a new force, but rather trying to ensure that local fighters were able to protect liberated areas from attacks by IS remnants.
Turkey’s president branded it a “terror army” and warned of imminent assaults on the Kurdish-controlled border areas of Afrin and Manbij.
The Turkish army opened gaps in the border fence west of Afrin on Thursday, as the state-run Anadolu news agency reported that troops had been put on high alert.
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Why does the US want to stay in Syria?
In a speech at Stanford University on Wednesday, Mr Tillerson said decisive action taken by President Trump had accelerated the progress made against IS.
But he noted that IS was “not completely defeated” and that the government of President Bashar al-Assad – who the US opposes and has said should step down from power – controlled about half of Syria’s territory and population.