"There has to be a full restoration both of the institutions of governance and justice as well as the full restoration of a duly democratically elected civilian government," the senior State Department official told journalists.
It follows a recent visit by Mr Daniel Russel, the most senior US official to travel to the kingdom since Thai generals imposed martial law and took over in a coup last May.
The United States strongly condemned the coup at the time, with chief diplomat John Kerry calling for a return to civilian rule via democratic elections.
Washington and Bangkok maintain a military alliance dating to the Cold War and the fight against communism in Southeast Asia, notably during the Vietnam War.
US and Thai armed forces remain tightly linked, but the United States canceled joint military exercises after the coup and suspended a small part of its military assistance to Thailand.
© 1994-2015 Agence France-Presse