Image caption The bill was passed unanimously but in the absence of opposition politicians

Cambodia's parliament has approved a bill which makes it illegal to deny that atrocities were committed by the Khmer Rouge in the 1970s.

The move comes amid political in-fighting in the run-up to elections.

Anyone found guilty of denying or playing down the crimes could face up to two years in prison.

About 1.7 million people, about one-third of the population, are thought to have been killed, or died of over-work, starvation or torture from 1975-1979.

Prime Minister Hun Sen proposed the law after an opposition leader apparently blamed Vietnam for some of the deaths and reportedly said that the infamous S-21 torture prison "was staged". The lawmaker maintains that the recording of his words was doctored.

The Vietnamese invasion in January 1979 shattered the Khmer Rouge's leadership in Cambodia.

Who were the Khmer Rouge? Maoist regime that ruled Cambodia from 1975-1979

Led by Saloth Sar, better known as Pol Pot

Abolished religion, schools and currency in effort to create agrarian utopia

Up to two million people thought to have died of starvation, overwork or by execution

Defeated in Vietnamese invasion in 1979

Pol Pot fled and remained free until 1997 - he died a year later

'Election-related'

The bill was drafted within a week and passed unanimously but in the absence of all opposition politicians. They were expelled from parliament after forming a new party, the Cambodia National Rescue party.

The party said it was "disappointed" by the move because the expulsion of its lawmakers had left parliament without a quorum, the Associated Press news agency reported.

Critics say the denial law runs the risk of being used as a weapon against the political opposition.

Human Rights Watch's Asia director Brad Adams said Prime Minister Hun Sen's advocacy of the law was "entirely election-related".

"It's a tool to try to intimidate the opposition but also to galvanise his side, to demonise the opposition as unfit to govern, and to show that he's in charge, to show the country that he can completely dominate the opposition - and make them squirm," he said.

The Khmer Rouge regime fell in 1979, and the process of trying its senior figures has taken many years.

The only former Khmer Rouge leader to have been successfully prosecuted was chief jailer Kaing Guek Eav, known as Duch, who was sentenced to life imprisonment for his role in running the notorious Tuol Sleng prison, where thousands of inmates were killed.