PHNOM PENH (Reuters) - Vaguely worded Cambodian media rules could bring sweeping restrictions that prevent journalists reporting on controversial issues in the run-up to a general election, U.N. rights monitors warned on Friday.

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The government of veteran Prime Minister Hun Sen, who aims to extend his rule in the July 29 vote, has mounted a broad campaign against the opposition and dissent and said this month it was watching out for any online news that causes “instability”.

Authorities have also promised to “control news content” while the National Election Committee has issued a code of conduct banning journalists from expressing opinions and from publishing news that affects national security. Journalists have also been barred from conducting interviews at polling stations on election day.

“These prohibitions use broad and imprecise terminology that could lead to sweeping restrictions on the media that would be incompatible with international standards,” Rhona Smith and David Kaye, both U.N. Special Rapporteurs, said in a statement issued in Phnom Penh.

“Journalists have a responsibility to report on many issues in the run-up to an election, in particular controversial issues,” they said.

The rapporteurs urged Cambodia to respect press freedom.

A spokesman for the election committee, Hang Puthea, dismissed the concerns, saying media guidelines were aimed at ensuring the election goes smoothly.

The opposition Cambodia National Rescue Party (CNRP), which was dissolved by a court last November at the government’s request, has urged voters to boycott the vote.

The U.N. experts said voting was not compulsory.

“We encourage the Cambodian authorities to ensure that all people are freely able to express their political views and opinions, including on the option of abstaining,” they said.

The CNRP leader is in jail, on what he says is a politically motivated charge, while most of his senior colleagues are in exile, raising concern among aid donors in the West that some 25 years of effort to build democratic institutions have come to nothing.