Fortify Rights said in a report that four soldiers detained Aung Ko Ko, 37, in Thailand's western Mae Sot district on January 12 for wearing the uniform of an official Thai village security force.

Three of them beat Aung Ko Ko with a long wooden stick while interrogating him, leaving him bruised and bleeding nearby until he died hours later, the rights group said, citing eyewitness accounts, photos of the scene and an autopsy report.

"The horrific torture and killing of Aung Ko Ko cannot be allowed to go unanswered. The soldiers responsible for this should be brought to justice without delay," said Matthew Smith, CEO of Fortify Rights.

Another Myanmar national who witnessed the violence was later convicted of manslaughter over the death, the report said.

Sirachuch, 24, who goes by one name, was sentenced to three years and four months in prison, Fortify Rights said, condemning it as a "miscarriage of justice" that must be put right.

"We believe that Aung Ko Ko's tragic death highlights an ongoing pattern of impunity in Thailand for violence committed against migrants and refugees," the NGO said in a statement.

The army said it was looking into the allegations.

"We are in the process of investigating and looking for the facts, but Thailand has always given priority and give importance to human rights, equally to everybody," army spokesman Major General Thanathip Sawangsang told AFP.

An ongoing conflict in Myanmar, sparked by the military's 2021 coup, regularly sends people rushing across the 2,400-kilometre (1,490-mile) border between the two countries.

Around 90,000 refugees live in nine camps on the heavily patrolled Thai side of the border, according to the United Nations, many having escaped fighting between Myanmar's military and ethnic minority rebel armies.

Thai security forces have been criticised in the past for pushing boatloads of ethnic Rohingya entering Thai waters back out to sea, and for holding migrants in overcrowded facilities.

Thailand, which is not a signatory to the UN Refugee Convention, does not distinguish between refugees and other migrants, and thousands of people live under the radar in the country.