PHNOM PENH: The Cambodian government is choking freedoms and locking up detractors in an increasingly bold effort to silence critics as elections loom, observers say.

Prime Minister Hun Sen, 59, who has vowed to remain in power until he is 90, recently said on national radio that his aim was "not just to weaken the opposition, but to make it die".

The comment was the latest in a string of outbursts against critics, prompting fears that freedoms are under threat as the government looks ahead to local polls next year and a general election in 2013.

"The space for dissent has shrunk to the point where people are gasping for air," said Mathieu Pellerin of local rights group Licadho.

"Vast areas of political debate have been effectively declared off-limits. The most minor venture into these fenced-off topics can bring the authorities' wrath, whether you are a prominent politician or an anonymous village farmer."

Outspoken opposition leader Sam Rainsy, who lives in self-imposed exile, has been sentenced in absentia to 12 years in jail over two cases related to border issues with Vietnam.

If the sentences are upheld, he will be unable to challenge Hun Sen's ruling Cambodian People's Party (CPP) in the 2013 poll.

"The CPP is preparing for the next election, that much is clear," said a Cambodia-based Western expert, on the condition of anonymity.

"To do that, they want to reduce as much as possible any public criticism that would cost them ballots."

Dismissing concerns about a crackdown on freedoms, government spokesman Tith Sothea said the government was "working to protect human rights and carry out reforms in order to ensure political stability".

Mark Turner, a Cambodia expert at the University of Canberra, said the legacy of the country's recent bloody history has allowed the ruling party to tighten its grip on power.

"One of the leading themes of post-Khmer Rouge Cambodia has been the search for stability," he said. "If incomes are rising, education improving, health facilities more accessible, then people may accept a certain curtailment of freedoms."

Cambodia remains haunted by its past, after decades of civil war and the brutal 1975-1979 Khmer Rouge regime that left up to two million dead in its bid to forge a communist utopia.

Cambodian independent analyst Chea Vannath said it was important to recognise how far the nation had come considering its "terrible past".

Hun Sen, who has ruled since 1985, has been credited with the country's long spell of peace and stability, while also improving infrastructure and opening up the country's markets.

But he also has a history of riding roughshod over his rivals, and analysts say the CPP -- bolstered by a 2008 election landslide -- has exerted executive power without limits.

It is now a crime to criticise judges or public officials under a new penal code that activists say could be used as a government tool to muzzle freedom of expression.

"Impunity is deepening for government power-holders and their cronies to abuse rights," said Human Rights Watch deputy Asia director Phil Robertson.

"At the core of all of this is the continued lack of independence of the Cambodian judiciary, which suffers endemic political interference from the CPP and other governing elites."

One of the first to be arrested under the new code was a World Food Programme worker, sentenced to six months in prison for incitement after he printed an article from an anti-government website.

The government has mounted what Robertson terms a "campaign of intimidation" against the UN in Cambodia, threatening to expel the organisation's resident coordinator Douglas Broderick after he called for more transparency in the debate about a new anti-corruption law.

The government also used a high-profile visit by UN chief Ban Ki-moon to demand the removal of local human rights director Christophe Peschoux.

Foreign Minister Hor Namhong said Peschoux had acted as "the spokesman for the opposition", after the Frenchman spoke out on issues such as land-grabbing and crackdowns on government critics.

Despite steady economic growth Cambodia remains one of the region's poorest nations, presenting foreign donors with an opportunity to defend those that have come under attack, activists said.

Outside aid contributed around one billion dollars, or about nine percent of Cambodia's economic output in 2010.

"Donors need to wake up and recognise the human rights situation in Cambodia is rapidly deteriorating," said Robertson.