Cambodia on Saturday denied it had banned foreign radio broadcasts in the run up to next month's election, after the US accused the government of violating freedom of expression.

This week, local FM radio stations were ordered to provide "neutral" coverage of election campaigning and to temporarily suspend broadcasting Khmer-language programs made by foreign stations.

But Chhum Socheat, an official at the information ministry, told AFP on Saturday "we do not ban broadcast by foreign radios".

Foreign radio stations can still air their programmes on short wave transmissions, he said.

He added that the directive, asking local FM radio stations not to air Khmer-language programmes produced by foreign radio until after the July 28 election, was to allow for "fair campaigning" for all political parties.

The directive, released late Friday, also barred "foreigners in Cambodia from campaigning in favour or against any political party" and said that "legal action" would be taken against local FM stations that did not comply.

The move was attacked by the US, who said it was a "serious infringement on freedom of the press and freedom of expression" and by broadcasters including US-funded Radio Free Asia.

The move is "the most sweeping and stunning frontal assault on media freedom in Cambodia in recent memory," RFA said in a statement posted on its website.

It is "a blatant strategy to silence the types of disparate and varied voices that characterize an open and free society," the broadcaster, which produces shows in the Khmer language, added.

Cambodia on Thursday officially started campaigning for the July 28 general election, expected to be won by strongman Prime Minister Hun Sen who is seeking to extend his 28-year grip on the country.

Hun Sen has run Cambodia for 28 years, making him Southeast Asia's longest-serving leader beside the sultan of Brunei.

His government is regularly accused of suppressing political freedoms and muzzling activists.

Opposition leader Sam Rainsy, his main challenger, is barred from running in the polls due to a string of convictions that the opposition says are politically motivated.

Rainsy, who lives in exile in France to avoid prison, faces 11 years in jail if he returns, after he was convicted in absentia for charges that included publishing a "false map" of the border with Vietnam.

The idiosyncratic Hun Sen last month said he would try to stay in power for more than a decade, until he is 74. He previously vowed to hold office until he reaches 90. (AFP)