BANGKOK (Reuters) - A Thai military court issued arrest warrants for three people on Wednesday (Nov 25) as part of a sweeping royal defamation investigation which has seen seven people charged so far, including two who died in custody.

The three men were arrested over alleged computer crimes and for defaming the monarchy. The military did not say whether the three were involved in the wider royal defamation probe.

The deaths of two of the suspects detained so far have raised serious questions about the investigation and prompted the United Nations on Tuesday to call for the immediate closure of a Bangkok military detention facility where the two men died.

All seven people have been charged under Thailand's lese majeste law which is the harshest such law in the world and states that "whoever defames, insults or threatens the king, queen, heir-apparent or regent shall be punished with imprisonment of three to 15 years".

They have been accused of a range of offences including using the monarchy's name for personal gain in relation to two nationwide cycling events aimed at honouring the king and queen.

The three charged on Wednesday include two former policemen and a former military officer, a senior police officer involved in the investigation told Reuters.

The two men who died in custody were Prakrom Warunprapa, who authorities said hanged himself in his cell last month, and Suriyan Sucharitpalawong, a famous soothsayer, who died from a blood infection, according to the corrections department.

Since taking power in a coup last year, the ruling junta under Prime Minister Prayut Chan-o-cha has made hunting down and prosecuting people deemed to be critical of the monarchy a national priority.

But rights groups have criticised the fervour with which the junta has gone after perceived royal critics, saying the law is being used as a tool to crush political opponents of the junta.

The investigation comes at a time of heightened anxiety over the health of 87-year-old King Bhumibol Adulyadej, who is convalescing at a Bangkok hospital after being treated by doctors for "water on the brain", which has added to the political uncertainty shrouding Thailand since the coup.