Cambodia's Prime Minister Hun Sen says the government is considering suing a cousin of reigning King Sihamoni, after he was critical of the government's border policies.

Speaking on a television and radio broadcast Hun Sen said officials are considering whether to sue prince Sisowath Thomico.

A statement bearing the prince's name but not his signature, claiming the premier had given Koh Tral island to Vietnam in 1982, was broadcast on the Voice of America radio service and posted on a web site former King Norodom Sihanouk, a long time critic of Hun Sen.

If the lawsuit is filed, it would mark the latest in a serious of moves the government has taken against its critics, who charge Phnom Penh has given up or sold land to Cambodia's neighbours.

The prime minister said last week the island became Vietnam's in 1939, before he was born.

On Monday, he said he would resign and willingly face a court trial if he willingly relinquished Cambodian land.

"The case of selling land is very serious," he said. "It is a case of betraying the nation."

Hun Sen has warned he would sue anyone accusing him of selling land to foreigners and said foreign countries should not interfere in the kingdom's internal affairs.

At least six people, including a teachers' union leader and the head of a radio station, have been arrested or been the target of lawsuits for questioning the government's border policies.

ABC Asia Pacific TV / Radio Australia