BANGKOK (AFP) - The mutilated bodies of two men found stuffed with concrete in the Mekong River last month have been identified as Thai activists living in exile, police and family confirmed on Tuesday (Jan 22).

The pair were among dozens of anti-monarchy activists who fled Thailand to communist Laos after a 2014 coup by the staunchly royalist junta.

Surachai Sae-Dan, a prominent critic of Thailand's monarchy and the Thai junta, went missing on Dec 12 with his aides, Kraidej Luelert and another man known only as "Comrade Phuchana".

Later that month, two bodies were found floating in the river in north-eastern Thailand, their faces maimed and stomachs disembowelled and stuffed with concrete blocks.

DNA forensic testing had concluded there was a "99.95 per cent" chance one of the bodies was Kraidej after a comparative analysis with his son, said district police chief Kiattipoom Suwannatrai.

Phuchana's son said the forensics department in Bangkok had confirmed his father was the other victim, while another police source who requested anonymity due to the sensitivity of the case told AFP the second body was Phuchana.

Surachai's whereabouts are still unknown.

Human Rights Watch said five opponents of Thailand's monarchy living in Laos have gone missing in the past two years, suggesting other activists in exile were in "serious" danger.

The Laos government should conduct "thorough investigations on Surachai's whereabouts and on who was responsible for the death of the two activists", HRW senior researcher Sunai Phasuk said.

Thailand has some of the world's strictest royal insult laws and the junta has tried in vain to extradite a number of prominent anti-monarchy activists who fled abroad after the 2014 coup.

The country is slated to hold long-delayed elections this year followed by the coronation of new king Maha Vajiralongkorn, who will assume the throne two-and-a-half years after the death of his highly revered father.