BANGKOK (AFP) - Thailand's military faced mounting pressure on Friday (Nov 24) to explain the murky death of a teenage army cadet, whose case has seized national attention after his parents discovered his organs were mysteriously removed from his body.

The army said first-year cadet Pakapong Tanyakan died of heart failure late October at his army training school outside Bangkok.

But his family was sceptical and ordered a second autopsy - only to find their son's brain, heart, bladder and stomach were missing.

The shocking discovery sparked public outcry and accusations of a cover-up by a military trailed by allegations of beatings and other abuse against young recruits that can turn fatal.

The army has continued to deny foul play in Mr Pakapong's case, saying doctors removed his organs for further inspection and were not required to inform his family.

But thousands in the junta-run nation have signed an online petition calling for the resignation of the cadet's commanders.

On Friday a government spokesman said four military officers had been transferred while an investigation is ongoing.

"To help cope with all sides' uneasiness, the Royal Thai Armed Forces Headquarters has transferred officers who are involved with this case so that they will not interfere with the evidence or witnesses," Lieutenant-General Sansern Kaewkamnerd told AFP.

He added that junta chief Prayut Chan-O-Cha was "deeply sorry" to hear about the cadet's death.

The case has unleashed an outpouring of grievances online, with netizens calling for an end to corporal punishment in the military.

"My son wanted to be a soldier but after seeing this news, he lost his confidence," Bumbim Kanyaplak posted on Facebook.

Mr Pakapong's family told reporters their son had described physical abuse throughout his time at the school, including a hazing exercise that led him to faint in August.

He also described being punched in the stomach in a diary entry in May.

Thailand's military, which has run the country since a 2014 coup, runs an annual draft that brings 100,000 men into the armed forces every year.

There have been at least three reported cases of conscripts dying during training this year.

Speaking to the press earlier this week, Defence Minister Prawit Wongsuwan admitted that physical punishment is a part of military training, saying he had gone through the ordeal himself.

"But I didn't die," he added.

Mr Prawit apologised for the remarks on Friday, saying they were insensitive to the cadet's family.