The American ambassador to Cambodia on Tuesday rejected allegations by strongman premier Hun Sen that the US was involved in plotting to overthrow the government, calling the claims a "red herring".

The accusations followed the arrest of leading opposition politician Kem Sokha, who was charged last week with treason and espionage in a "secret plan" with foreigners.

Hun Sen publicly accused the US of involvement in the plot, citing a 2013 speech in which Kem Sokha said he had received American help to build a pro-democracy movement inside Cambodia.

The finger-pointing comes as Hun Sen dramatically escalates his rhetorical attacks on the US -- an ally whose foreign aid has become less vital as Cambodia edges closer to China.

"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 withering statement.

Tall tales about US plots with Kem Sokha "have completely, and intentionally, mischaracterised what the United States is doing in Cambodia," he added, calling them "classic red herrings".

"Honestly, the whole thing is just absurd," he added in unusually frank remarks.

Over the past week the embassy's social media accounts have been publishing images of a red fish, along with the hashtag #RedHerringsKH, beneath posts about US aid projects in the kingdom.

The ambassador called for Kem Sokha's immediate release and said his arrest had imperilled Cambodia's fragile democracy.

Washington and Phnom Penh have a complex history.

The US pummelled the country with bombs during the Vietnam War and then became one of its biggest donors as the kingdom rebuilt from the ashes of the brutal Khmer Rouge era of 1975-79.

But its influence has waned significantly as China muscles into Southeast Asia, enticing authoritarian leaders like Hun Sen with aid and investment free of pressure to safeguard human rights.

Kem Sokha's dramatic arrest was only the latest blow to his Cambodian National Rescue Party, the only party offering a viable alternative to Hun Sen in next year's election.

After almost losing to the opposition party in 2013, Hun Sen's government has moved systematically to silence his opponents in politics, the media and rights groups.

The self-described strongman has ruled Cambodia for 32 years and vowed last week to stay on for another ten.

Source: AFP