Thailand's military courts have issued the harshest sentences in recent decades for two people convicted of insulting the monarchy on Facebook.

A man in Bangkok was sentenced to prison for 30 years, and a mother of two young girls was sentenced in northern Thailand for 28 years.

The watchdog group iLaw said the Bangkok Military Court sentenced Pongsak Sriboonpeng on Friday to 60 years imprisonment, 10 for each message, but halved his sentence because he admitted wrongdoing.

The hotel worker in the northern province of Chiang Mai was also found guilty of posting offensive Facebook comments, but had her cumulative sentence halved to 28 years after pleading guilty.

Thailand's lese majeste law mandates prison terms of three to 15 years for defaming, insulting or threatening the monarchy.