United States Senator Ted Cruz issued an ultimatum to Prime Minister Hun Sen yesterday, pledging to work to ban top officials from travelling to the US unless jailed opposition leader Kem Sokha is released.

In a letter to the Cambodian ambassador to the US yesterday, the Republican senator and former presidential candidate said he would work with Congress and the administration of Donald Trump to prevent certain government officials from travelling to the US unless Sokha – whose treason charges he said “appear to be political in nature” – is freed by November 9.

“From removing independent election monitors and human rights organizations to shuttering hundreds of radio stations imprisoning Kem Sokha, the Prime Minister . . . is endangering the future not only of U.S.-Cambodia relations, but of Cambodian democracy,” Cruz wrote. If Sokha remains behind bars, “it will be impossible for any important observer or nation to certify that elections in your country have been free and fair”, the letter continued.

Sokha was arrested and charged with treason in September after a 2013 video surfaced in which he claims to have received assistance from the US in planning his political career.

The ultimatum was brushed off by government officials yesterday.

“I don’t think that senator has the right to decide that,” said Ministry of Interior spokesman Khieu Sopheak, who laughed when informed about the letter. “I studied US law . . . He has no right to do that because he’s not the Trump administration,” he added.

Government spokesman Phay Siphan, who is a dual American-Cambodian citizen, said the letter was an insult to Cambodia’s sovereignty. “I’m proud to be Cambodian,” Siphan said. “I don’t trade anything from my Cambodian heritage just to travel to the United States.”

Asked whether the national election next year can be fair with the only viable opposition party on the brink of dissolution, Siphan said, “Our nation is based on Cambodian choice.”

“We don’t defer to make the foreigner happy,” he said.

Cruz joins a trickle of US congressmen who have spoken out about increasing political oppression in Cambodia. US Senator John McCain and representatives Alan Lowenthal and Steve Chabot have also been vocal about their criticisms of Hun Sen in recent months.

As of September, top Foreign Affairs Ministry officials are banned from obtaining visas to the US in response to the Kingdom’s refusal to accept Cambodian-American deportees.