The accusation comes after Cambodian authorities arrested leading opposition politician Kem Sokha September 3 and charged him with treason in connection with an alleged plot to work with the U.S. to depose Prime Minister Hun Sen.
The government in Phnom Penh said the charges against Kem Sokha, of the Cambodia National Rescue Party, or CNRP, were based on comments he made in 2013, in which he claims to have received help from the U.S. to build a pro-democracy movement.
Ambassador William Heidt said in a statement that stories about U.S. plots involving Kem Sokha "have completely, and intentionally, mischaracterized what the United States is doing in Cambodia."
"Honestly, the whole thing is just absurd," he said in his statement. Heidt also called for the immediate release of Kem Sokha, adding that his arrest threatened Cambodia's fragile democracy.
FILE - Opposition party Cambodia National Rescue Party (CNRP) President Kem Sokha adressess party supporters during the party';s political congress in Phnom Penh, Cambodia, March 2, 2017.
The CNRP has said it will contest next year's election, despite Kem Sokha's arrest and threats from Hun Sen to dissolve the opposition party if it intervenes on Kem Sokha's behalf. At a news conference Tuesday, Son Chhay, a senior CNRP member, pledged that the party would challenge the ruling Cambodian People's Party, or CPP in 2018.
Kem Sokha's arrest was carried out amid a massive government crackdown against independent news outlets and human rights groups. Last Monday, The Cambodia Daily, one of the last independent newspapers in the country, was closed after it received a large, overdue tax bill its publishers claim is bogus.
Observers say the crackdown is an apparent attempt by Hun Sen to shut down dissenting voices ahead of next year's election with the aim of extending his three-decade-old grip on power.
Hun Sen's government was nearly toppled in the last national election in 2013, and support is growing for the opposition, especially among younger Cambodians.