The U.S. ambassador to Cambodia rejected government accusations of interference by the United States as “inaccurate, misleading and baseless” on Tuesday and called for the release of detained opposition leader Kem Sokha. It was the strongest U.S. response since the Sept. 3 arrest of Kem Sokha, who has charged with treason and accused of plotting with the United States to take power from Prime Minister Hun Sen, a former Khmer Rouge commander who has ruled Cambodia for over 30 years.

Hun Sen, now one of China’s closest regional allies, has stepped up rhetoric against the United States alongside a crackdown on opponents, independent media and other critics ahead of a general election next year.

“On dozens of occasions over the past year, the United States has been subject to intentionally inaccurate, misleading and baseless accusations,” Ambassador William Heidt said in a statement. “All of the accusations you have heard in recent weeks about the United States – every one of them – are false.”

Heidt called for the release of Kem Sokha, the end to pressure on civil society and for dialogue between the government and opposition to “salvage” elections and restore the bilateral relationship between the countries.

 Cambodia election

“If Cambodia’s national elections were held today, no credible international observer would certify them as free, fair and reflecting the will of the Cambodian people,” he said.

American and Western companies were feeling less welcome in Cambodia “and fewer will invest”, he said.

Hun Sen, 65, has shown no sign of softening his line of seeking dialogue. On Monday, he threatened that Kem Sokha’s Cambodia National Rescue Party (CNRP) would dissolved if it continues to back him. Kem Sokha, 64, is the only serious election rival to Hun Sen, who could face his biggest electoral challenge next year.

In a news briefing on Tuesday, senior CNRP member Son Chhay said the opposition would not boycott the July 2018 general election in which it faces Hun Sen’s ruling Cambodian People’s Party (CPP).

“The CNRP will go into the 2018 election despite the enormous difficulties,” Son Chhay said.

The evidence presented against Kem Sokha so far a video recorded in 2013 in which he discusses a strategy to win power with the help of unspecified Americans. His lawyers have dismissed the evidence as nonsense and said he was only discussing election strategy.

On Tuesday, Son Chhay said Kem Sokha was innocent until a final conviction by court, saying that the arrest and a parliament vote to allow his prosecution were illegal.