US Secretary of State Anthony Blinken met a Cambodian opposition leader facing treason charges during a visit to Phnom Penh Friday, expressing concern about the kingdom's eroding democracy ahead of elections next year.

One of the world's longest-serving leaders, Prime Minister Hun Sen has ruled Cambodia for more than 37 years. In 2018 his party won every seat in a national election.

And in June, a Phnom Penh court convicted about 60 opposition figures -- including dual US-Cambodian citizen Theary Seng -- in a mass treason trial that rights groups have condemned as politically motivated.

Blinken said he directly expressed concerns about the kingdom's ailing democracy during talks with Hun Sen, while visiting the Cambodian capital for the Association of South East Nations (ASEAN) foreign ministers' meeting.

Allowing broader participation in the political system would strengthen the country, the secretary of state told reporters Friday.

"One of the things that stands in the way of strengthening the relationship between our countries are our concerns about eroding respect for human rights and democratic institutions here in Cambodia," he said.

Washington's top diplomat said that earlier Friday he met Cambodian opposition leader Kem Sokha, who is on trial for treason in a case that has dragged on since January 2020 -- and which could effectively rule him out of next year's national election.

During talks with Hun Sen, Blinken also raised the case of Theary Seng. The outspoken lawyer and campaigner was hit with a six-year jail term in June's treason case.

Hun Sen and his ruling party have publicly backed the prime minister's eldest son -- currently commander of the army -- to eventually take the reins.

Ahead of local elections two months ago, the United Nations Human Rights Office said it was disturbed by reports of opposition obstruction in a "paralysing political environment".

Hun Sen's political party later announced a landslide win.

AFP