US umbrella union group AFL-CIO said Thailand had breached standards under the Generalised System of Preferences (GSP),which allows duty-free access for some imports, on a range of issues including acceptable conditions of work and forced labour.
USTR said in a statement it would hold a hearing in January on Thailand, and continue its work on GSP reviews of Fiji, Georgia, Iraq, Niger, Uzbekistan and Ecuador.
Thailand exported $27.1 billion in goods to the United States in 2014, mainly machinery and electrical machinery.
USTR closed a review of the Philippines after the country made progress in protecting workers' rights.