U.S. Secretary of States John Kerry called for Cambodia's government to allow "vigorous and peaceful" political debate ahead of elections in 2018 and said progress on human rights was critical to future relations.
On a visit to Phnom Penh, Kerry met Hun Sen, Asia's longest-serving prime minister, and Foreign Minister Hor Nam Hong. He also met leaders of the opposition and civil groups to underscore U.S. concerns about human rights.
With three years to go before the vote, political tension in Cambodia is already high. Hun Sen's biggest rival, opposition leader Sam Rainsy, fled to France in November to avoid arrest for an old defamation case which Sam Rainsy's political party say was revived at Hun Sen's bidding.
Lawmakers from Sam Rainsy's party, the Cambodian National Rescue Party, were beaten in broad daylight outside in October outside the national assembly.
"In my discussions today, I emphasized the essential role that a vibrant, democratic system plays in the development of a country and the legitimacy of its political system," Kerry said.
"Democratic governments have a responsibility to ensure that all elected representatives are free to perform their duties without fear of attack or arrest."
Kerry said discussions required directness and candour, even on sensitive issues such as human rights.
"We care deeply about respect for human rights, universal freedoms, and good governance," he said. "Progress in each of these areas is critical being able to fulfill the potential of our bilateral relations but, also importantly, the full potential of the hopes and aspirations of the Cambodian people."
Robust economic growth, job creation and sustained peace for an impoverished country devastated by decades of civil war, including under Pol Pot's 1975-79 "killing fields" regime, have ensured Hun Sen's continued re-election.
Pol Pot and his Khmer Rouge swept to power after Cambodia was heavily bombed by the United States during the Vietnam War.
Kerry called Cambodia's recent economic progress "quite remarkable".
"You have moved from extreme poverty, from a very small economy, to an economy that is growing very significantly and lifted many of your people out of poverty," he told Hor Nam Hong.
Kerry also visited Cambodia's National Museum to demonstrate U.S. support for efforts to protect the country's cultural heritage through the return of illegally exported artifacts.
Several significant cultural objects taken out of Cambodia illegally have been returned by U.S. private collections under an agreement between the two governments.