The United States expressed concern Monday after Thailand approved a new military-backed constitution in a victory for the country's ruling generals.
The bitterly divided kingdom has been ruled by a junta for two years since its elected government was booted from office.
Sunday's majority "yes" vote in support of the charter was the first test of public opinion since the 2014 coup, although independent campaigning and open debate was stifled ahead of the polls.
US State Department spokeswoman Elizabeth Trudeau said the United States remains "concerned that the drafting process for the constitution was not inclusive, that open debate was not permitted in the run up to its adoption."
"We urge Thai authorities to proceed with next steps to return Thailand to elected civilian-led government as soon as possible," she said.
Washington and Bangkok maintain a military alliance leftover from the Vietnam and Cold War periods, but relations have grown tense since the coup two years ago.
Since then the United States has started looking to strengthen ties elsewhere in Southeast Asia like Singapore, Malaysia, the Philippines and Vietnam.
Washington regularly calls on Thailand to organise new legislative elections and reestablish democratic institutions.
"We strongly urge the government to lift restrictions on civil liberties, including freedom of expression, the right to peaceful assembly," Trudeau said. "Then Thai people can engage in an open unimpeded dialogue about the country's political future."
