PHNOM PENH (Reuters) - Cambodia's military is investigating a suspected coup plot against Prime Minister Hun Sen, a Defence Ministry official said on Wednesday (July 20), amid rising political tension in the South-east Asian country.

The army investigation centres on a man who announced plans to topple Mr Hun Sen in Facebook and YouTube videos and called on others to join his cause, said Defence Ministry spokesman Chhum Sucheat. "The authorities are investigating and looking for him," Mr Chhum Sucheat told Reuters. He identified the man as Vichea Som but declined to give further details.

News of the plot comes a day after a senior United States official responsible for human rights urged Cambodia's political rivals to return to negotiations amid a "deteriorating situation".

In the video, the man the ministry identified as Vichea Som is dressed in a business suit and tie with a camouflage background visible in the footage.

He did not identify himself but accused Mr Hun Sen's government and his ruling Cambodian People's Party of being behind big land grabs in one of South-east Asia's poorest countries. He also accused the government of high-profile killings and human rights abuses.

"In the near future, please all forces be ready against the dictatorial regime," the man said.

He did not specifically call for a coup or threaten to organise a coup in the video clip.

It was not clear where he was speaking from but he said he belonged to a "unit" based in south-western Cambodia.

The Facebook page bearing the name Vichea Som showed the same man. He did not immediately respond to a request for comment sent via Facebook.

Mr Hun Sen has ruled with an iron fist for more than 30 years, defeating all challenges to his authority, but he faces a young electorate which appears increasingly hungry for change.

In recent moths, tension has been rising between Mr Hun Sen and an opposition hoping to challenge his grip in local elections next year and a general election in 2018.

Several members of the opposition and activists have been jailed as part of what they say is a government crackdown to silence critics ahead of the elections.

On July 10, Mr Kem Ley, a prominent activist and frequent critic of Mr Hun Sen, was murdered in the capital Phnom Penh.

A suspect has been arrested and the police said they believed Mr Kem Ley was killed because of a debt. Activists believe his murder was political.

The United States has called for a credible investigation. The European Union and United Nations have expressed concern about the political tension.