Thailand's powerful army chief expressed opposition Tuesday to amending the country's lese majeste law, recently criticised by the West, saying those who opposed it should move abroad.

Critics say that Thailand has suppressed freedom of expression with stepped up use of the legislation, under which anyone convicted of insulting the king, queen, heir or regent faces up to 15 years in prison on each count.

"It is not appropriate to discuss this. Personally I will do my part for national security in protecting (the monarchy)," General Prayut Chan-O-Cha told reporters.

When asked about activists calling for reform of the law, he said they "must go and live in foreign countries".

"Although we are a democracy, don't go too far," he added.

The United States, the European Union and the United Nations have expressed concern over recent convictions, which have also sparked small protests both for and against the law in the Thai capital Bangkok.

A 61-year-old Thai man was jailed last month for 20 years for sending text messages deemed insulting to the monarchy, while a US citizen has since been handed two-and-a-half years in prison for defaming the king.

The convictions intensified debates on the issue on social media websites, although a committee was set up earlier this month to clamp down on online insults to the monarchy.

Despite mounting rights concerns, deputy prime minister Chalerm Yubamrung was also on the defensive when asked about the law on Tuesday.

"I think there is no need for any changes. Why do we need change when the law is already good?" he said.

"I don't want to comment on this issue, but whoever wants to amend this law, their lives will not be prosperous," he told reporters at Government House.

Last week, the US embassy in Bangkok requested users on its Facebook page to refrain from abusive language, after a torrent of angry messages from Thais defending the law and the monarchy.

About 100 royalists also gathered in front of the US embassy on Friday in support of law following the criticism from Washington.

Opponents of the law say it has increasingly been used in Thailand to stifle free speech, particularly under the last government, which was supported by the Bangkok-based elite.

Observers say the new government of Prime Minister Yingluck Shinawatra, who came to power in August, is yet to improve the situation, and there have been two rare protests against the laws this month.

Last week a "Red Shirt" activist was sentenced to 15 years in jail accused of defaming the royals during speeches at political rallies in 2008.