BANGKOK - A 14-year-old boy has been detained in a Thai army prison under a draconian and increasingly wielded royal defamation law, Human Rights Watch said Wednesday, as it decried the secrecy surrounding the latest series of arrests.

The law in Thailand forbids any criticism of the monarchy and punishes transgressors with up to 15-years in jail for each offence.

Use of the law has skyrocketed since an ultra-royalist junta seized power in 2014.

There have been a flurry of cases in recent weeks, many linked to social media posts about Thailand's new king Maha Vajiralongkorn, who ascended the throne late last year.

Royal insult cases are typically shrouded in secrecy with media forced to heavily self-censor details. But the latest detentions have been especially opaque.

Thai human rights groups and local media say a group of between four to seven people, including a minor, were arrested in northeastern Thailand on 15 May for allegedly burning a portrait of Vajiralongkorn's late father Bhumibol Adulyadej.

Thai Lawyers for Human Rights, which often provides legal representation to lese majeste suspects, said the group were taken from Khon Kaen province to a notorious army prison on a Bangkok barracks and that they had been held incommunicado since.

Thai police, military and junta spokespeople all declined repeated AFP requests for comment on the case.

In a statement on Wednesday, Human Rights Watch said their researchers had confirmed the names of four of the suspects -- 14-year-old Abhisit Chailee, Jirayu Sinpho, 19, Akkharapong Ayukong, 19, and Ratthathammanoon Srihabutr, 20.

Brad Adams, the group's Asia Director, said the latest arrests "should set off alarm bells".

"None of the four youths arrested should have been denied access to a judge and placed in an incommunicado military detention, whatever the charge against them," he said.

Thailand's junta has awarded itself powers to secretly detain anyone for up to seven days, but the current group has now been held for at least 10 days without access to lawyers or family.

More than 100 people have been charged with lese majeste since an army coup three years ago.

Late last year the United Nations' rights body warned Thailand's widespread use of the law "may constitute crimes against humanity".

But there has been no let up in its use. Thailand has seen an uptick in cases since the December ascension of Vajiralongkorn, who has yet to attain his father's popularity.

Earlier this month, seven people were arrested for allegedly sharing Facebook posts written by an exiled dissident academic. They were held incommunicado for days before appearing in court to be charged.

One of the suspects, human rights lawyer Prawet Prapanukul, was hit with a record 10 counts of lese majeste, a charge sheet that could see him jailed for up to 150 years.

This month Thai authorities also threatened to block Facebook if it did not do more to pull down critical or embarrassing commentary about the monarchy -- a vow the government later backed down over.-AFP