BANGKOK (REUTERS) - A Thai court on Wednesday (Jan 31) sentenced six people to jail for setting fire to portraits of King Maha Vajiralongkorn and his father, the late King Bhumibol Adulyadej, in several spots around the north-eastern province of Khon Kaen last year.

Thailand's lese majeste law is the world's toughest, and those judged guilty of breaking it face up to 15 years in jail for each count of offending the king, queen, heir or regent.

The six, aged between 18 and 20, were also found guilty of arson that damaged public property, as well as of organised crime, their lawyer Pattana Saiyai said.

"All of them confessed that they had no ill-intention, but were hired to go set fire to the royal portraits," Mr Pattana told Reuters, adding that the court took into account the lack of intent in reducing their jail terms.

One of the six was jailed for 11½ years, while three got terms of seven years and eight months each. Two 18-year-olds who set fire to only one portrait were jailed for three years and four months each.

The lawyer said police arrested two men in December over allegations that they paid his clients to set fire to the portraits, but the two have not yet been prosecuted.

Thailand's military, which took control of the government in a May 2014 coup, has said it needs to crack down on critics of the monarchy for the sake of national security.

Since the coup, at least 94 people have been prosecuted for lese majeste. As many as 43 people have been sentenced, says the iLaw group that monitors royal insult cases, with 92 per cent pleading guilty in hopes of shorter jail terms.

The laws protecting members of the royal family from insult limit what all news organisations can report from Thailand.

The United Nations has expressed concern over what it calls a deteriorating rights situation in Thailand, including harsh sentences for those convicted of violating the lese majeste law, known as Article 112.