BANGKOK (AFP) - A Thai man was sentenced on Wednesday (Jan 20) to six years in jail for two Facebook posts about the King that were deemed in breach of the country's draconian royal defamation law.

The conviction is the latest jail term handed down in an unprecedented lese majeste crackdown launched since arch-royalist generals seized power in a May 2014 coup.

Convictions have sky rocketed since the takeover with record breaking jail sentences, many for social media posts, as the authorities broaden their interpretation of the law.

Bangkok Criminal Court said 46-year-old Piya Julkittiphan was convicted for posting two pictures with messages in 2013 that risked making the public "disrespectful or unfaithful" to the monarchy.

"The judge sentenced him to nine years but he has given useful testimony during the investigation so the court commuted one third of that sentence to six years imprisonment," the court said in its verdict.

The court did not provide details on the content of the posts, as is commonplace in lese majeste convictions.

Even when details are known, media must heavily self censor when reporting on such cases to avoid falling foul of the same law.

Thai Lawyers for Human Rights described Piya as a former stockbroker who was first arrested in December 2014 and has been in custody ever since.

King Bhumibol Adulyadej, the world's longest-serving monarch, is shielded from any debate and criticism by one of the world's harshest royal defamation laws.

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

King Bhumibol remains widely revered in Thailand but has spent much of the last two years in hospital and is rarely seen in public.

Thailand's military styles itself as the champion of the monarchy and army-chief turned Prime Minister Prayut Chan-o-cha has vowed to root out all criticism of the royal family.

Analysts say uncertainty as the king's reign enters its twilight years is a major factor in the political chaos that has beset Thailand for much of the last decade, as competing elites jostle for influence.

Last year two people received 25 and 30 year sentences respectively, both over Facebook posts - record breaking jail terms that drew international condemnation.

The boundaries for what counts as a royal insult have also expanded dramatically.

Last month a man was arrested for allegedly making a satirical online remark about the king's favourite dog who recently died.

As of early December, at least 61 people have been prosecuted for lese majeste since the coup, according to local human rights group iLaw.

Lese majeste prosecutions almost always result in a conviction, often in secret courts. Many defendants plead guilty, which normally leads to a reduced sentence.

Most of those who fall foul of the law are regime critics but they also include multiple senior officials swept up in nebulous corruption probes - at least two of whom died in military custody last year.