BANGKOK (AFP) - Washington's envoy to Thailand on Wednesday expressed concern about the "unprecedented" prison terms handed down under the kingdom's draconian royal defamation law, saying no one should be jailed for peacefully expressing their opinion.

Recently appointed US ambassador Glyn T. Davies made the comments at a talk at the Foreign Correspondents' Club of Thailand in Bangkok as lese majeste prosecutions skyrocket under the ruling military junta.

"We're also concerned by the lengthy and unprecedented prison sentences handed down by Thai military courts against civilians for violating the lese majeste law," Davies told a packed audience, after expressing worries about the way criminal defamation laws are more widely being used to stifle public debate.

Thailand's King Bhumibol Adulyadej, 87 - the world's longest-serving monarch - is revered and perceived as a near-deity by many in the country.

He is also shielded by one of the world's strictest royal defamation laws, prosecutions under which have surged since the military seized power from an elected government in May 2014.

Davies, who has only been in his role for around nine weeks, stressed the deep respect and admiration the US held for the Thai monarch before asserting his point on the right to express opinions freely.

"We believe no one should be jailed for peacefully expressing their views and we strongly support the ability of individuals and independent organisations to research and to report on important issues without fear of retaliation," he said.

Under the law, anyone convicted of insulting the king, queen, heir or regent can face up to 15 years in jail on each count.

Since last year's coup, the military has stepped up its patrol of alleged lese majeste offences, especially on social media, which analysts say is being used to dole out harsher prison sentences.

In August the United Nations said it was "appalled" by the record jail sentences of 30 and 28 years handed to two Thais for royal defamation for "insulting" the monarchy on Facebook.

All Thai-based media routinely self-censor when reporting on the royals for fear of falling foul of the law.

Under the legislation, anyone can launch a complaint and the police are duty-bound to investigate, with critics of the law saying it is often used to pursue political opponents of the country's military and royalist elite.

Thailand is a longtime ally of the US, but the relationship between the two nations has been strained since last year's coup, which Washington strongly condemned.

Maintaining these ties has posed a delicate balancing act for the US, which is reluctant to isolate its old friend in the region.

On Wednesday, Davies reiterated the US call for a return to democracy in Thailand, but stressed he did not want to "come across as wagging a finger".

"Thailand has to do it on its own," he said.