Than Dar, the wife of slain journalist Par Gyi, stands in front of a family photograph showing herself, her husband and daughter posing with Aung San Suu Kyi, during a Reuters interview at her home in Yangon October 28, 2014. REUTERS/Soe Zeya Tun

YANGON (Reuters) - A group of activist organisations in Myanmar demanded the setting up of an independent commission on Wednesday to probe the killing of a journalist in army custody, saying the military had delayed reporting his death for almost three weeks.

A committee formed by the 46 organisations said the journalist, Par Gyi, was killed on Oct 4.

"The military waited 20 days to release the report on his death," Ko Ko Gyi, one of the members of the committee, said at a news conference.

"They had plenty of time to inform the family, but they didn't, and this is why we're calling for an independent investigation into the incident."

Par Gyi was arrested on Sept. 30 after completing a photo assignment documenting clashes between the military and the rebel Democratic Karen Benevolent Army (DKBA) in the east, the Myanmar-based Assistance Association for Political Prisoners (AAPP) has said.

The AAPP has disputed the statement released by the military, which said Par Gyi was shot when he tried to steal a gun from a soldier and escape after being detained because he was a member of an ethnic Karen rebel organisation.

His wife, Than Dar, has said she suspects he died while being tortured, leading the military to secretly bury his body.

Speaking at the news conference, Than Dar broke down while insisting that her husband had no affiliation with any rebel units.

"My husband was never involved in the KKO or DKBA," she said, referring to splinter ethnic armies operating in eastern Myanmar, adding that he was a freelance journalist.

After 49 years of autocratic military rule, a semi-civilian government took power in Myanmar in 2011 and ushered in sweeping political and economic reforms. But rights groups say the government has been cracking down on journalists over the past year, and they accuse the military of continuing to commit abuses.

"We are helping Than Dar to discover the truth about what happened to Par Gyi so that in the future other citizens won’t have to face such terrible things," said Ko Ko Gyi.

"This is not the first case like this - many things like this have happened before.

"According to the wishes of Than Dar, we will submit a letter to the Myanmar National Human Rights Commission demanding an independent investigation into the murder."

The police and the government have declined to comment on the case and so far there has been no word of any investigation into the killing.

Opposition leader Aung San Suu Kyi has also not commented on the killing of Par Gyi, who is a former democracy activist and worked as one of her bodyguards from 1988 to 1990. Par Gyi's wife said he was in touch with the Nobel Prize laureate until just months before his death and Suu Kyi has sent a letter of condolence to the family.

(Reporting by Paul Mooney and Aye Win Myint; Editing by Raju Gopalakrishnan)