A U.N. expert on freedom of speech called Monday for Thailand to tone down laws that prohibit defamation of the country's royal family, saying the legislation and punishments involved may violate an international rights treaty.

The statement by Frank La Rue came on the day an American man appeared shackled in a Bangkok court to plead guilty to charges of defaming the royal family.

Thai-born American Joe Gordon faces up to 15 years in jail for translating excerpts of a locally banned biography of Thailand's King Bhumibol Adulyadej and posting them online.

La Rue, the U.N.'s independent expert on freedom of opinion and expression, said the country's lese majeste laws were vague and the punishments overly harsh.

"The threat of a long prison sentence and vagueness of what kinds of expression constitute defamation, insult, or threat to the monarchy, encourage self-censorship and stifle important debates on matters of public interest," La Rue said.

"This is exacerbated by the fact that the charges can be brought by private individuals and trials are often closed to the public," he added.

La Rue said that under the International Covenant on Civil and Political Rights, which Thailand has been a party to since 1996, countries are allowed to impose only very clear and limited exceptions to free speech, such as to protect the reputation of individuals and safeguard national security.

"The Thai penal code and the Computer Crimes Act do not meet these criteria," he said. "The laws are vague and overly broad, and the harsh criminal sanctions are neither necessary nor proportionate to protect the monarchy or national security."

Thailand's ambassador to the U.N. in Geneva, Sihasak Phuangketkeow, said the government was conscious of the problem raised by the U.N. expert.

"The monarchy is a very important institution _ it's the pillar of stability and unity in Thailand. But we are aware of the concerns," he said, adding that a committee had been appointed to advise the government on how to better implement the law.