WASHINGTON (AFP) - The United States on Friday slammed a ban on foreign radio broadcasts by the Cambodian government in the run-up to next month's elections as a "serious infringement" on press freedom.

The Cambodian information ministry had published a directive banning broadcasts of foreign-produced radio programs for 31 days before the July 28 vote, Acting Deputy State Department Spokesman Patrick Ventrell said.

"This directive is a serious infringement on freedom of the press and freedom of expression and starkly contradicts the spirit of a healthy democratic process," he told reporters.

"We are deeply concerned by this action and urge the Royal Government of Cambodia to reconsider this decision."

Radio Free Asia (RFA) reported on Friday that their Khmer programs, as well as those of Voice of America and Radio Australia, would be barred under the directive.

Mr Ventrell warned the Cambodian government's decision cast doubt on the "intentions and the credibility of the electoral process" even though Cambodian officials have publicly said they aimed to have free and fair elections.

Official campaigning for next month's general election, which is expected to be won by strongman Prime Minister Hun Sen who is seeking to extend his 28-year grip on the country, began on Thursday.

While all political parties are free to canvass voters and hold public events, observers say there is little chance of unseating the incumbent Hun Sen and his Cambodian People's Party (CPP), which won the last two polls by a landslide despite allegations of fraud and election irregularities.