After months denying any wrongdoing, Myanmar has admitted its forces helped kill 10 Rohingya in custody in an apparent bid to blame a few rogue soldiers for what the global community alleges is part of an organised ethnic cleansing campaign.

Accounts of mass murder, rape and torture from the 655,000 Rohingya who fled Myanmar's army to Bangladesh have horrified the world.

Since the August crackdown the army vigorously denied any abuses, instead locking down access to Rakhine state and accusing critics -- including the UN -- of pro-Rohingya bias and spreading "fake news".

Then late on Wednesday it suddenly changed tune: an internal probe found four members of the "security forces" helped kill 10 Rohingya militant suspects at Inn Din village on September 2, leaving their bodies in a hastily-dug pit.

"It was found that the incident was not submitted to superior levels," it said.

The unprecedented acknowledgment, relayed on the Facebook page of the office of army chief Min Aung Hlaing, rippled out across the rights community that has spent months piecing together allegations of numerous atrocities.

"This grisly admission is a sharp departure from the army's policy of blanket denial of any wrongdoing," said James Gomez, Amnesty International's regional director for Southeast Asia and the Pacific.

"However, it is only the tip of the iceberg," he added, urging independent investigation into other allegations.

Some observers say the army move is an attempt to retake control of the story after rumours of extra-judical killings at Inn Din and the surrounding area began to seep out.

The admission came the same day as two Reuters journalists were formally charged by police in court with breaching the Official Secrets Act.

The pair, Myanmar nationals Wa Lone and Kyaw Soe Oo, have widely covered the military campaign in Rakhine although Reuters has declined to comment on whether they were specifically reporting on the mass grave in Inn Din.

"The military's admission demonstrates the culpability of soldiers and commanders in mass atrocities," said Matthew Smith of Fortify Rights, suggesting the arrest of the journalists was a clumsy effort "to cover their tracks".

Khin Zaw Win, of Yangon-based think-tank the Tampadipa Institute, echoed that theory, speculating the army "had no option but to come clean with it".

In December the US hit General Maung Maung Soe, who led the Western Command in the Rakhine operations, with targeted sanctions.

That punishment may have rattled the upper army echelons, speculated Khin Zaw Win about an institution renowned for denials and secrecy.

Cover-ups "could go bad for them... there could be sanctions against other generals," he said.

- Swords and guns -

Until Wednesday the army said its operations had killed around 400 militants in a legitimate defence of the country from Muslim militants intent on taking over Rakhine.

Its stance helped curdle a siege mentality inside Myanmar where the Buddhist-majority public has little sympathy for the stateless Rohingya.

Inn Dinn, in Maungdaw district, was engulfed by violence in late August.

On September 1 the 10 "Bengali terrorists" were apprehended after clashes pitting Rohingya against security forces and Rakhine villagers enraged by the murder of a local man by militants, the army chief's office said in its post.

"Bengali" is loaded term for the Muslim minority who are widely reviled inside Myanmar.

The 10 suspects should have been taken to a police station.

Instead after a night of "interrogation" at a school they were killed the next day by Rakhine villagers armed with swords and "four members of security forces" who "shot them with the use of guns".

Their remains were found on December 20.

The account chimes with refugee testimony collected in Bangladesh on the Inn Din incident.

"We were told that the 10 people had been arrested," Chris Lewa, director of the Arakan Project, told AFP. "They were never heard of again."

The army said it would take action against the villagers and security forces involved -- and any officers who failed to control their subordinates.

Rights groups have seized on the army's own account of Inn Din as validation of the reams of testimony of multiple massacres.

But with Rakhine locked off to media, most aid agencies and UN investigators, there are calls for more leverage to be used on Myanmar -- more so as the repatriation of a limited number of refugees is soon due to begin.

The European Union in Myanmar said the killings confirm the need for a thorough and credible investigation into all violence incidents in northern Rakhine state "to ensure the accountability of those found responsible for committing atrocities".

Myanmar should be reported to the International Criminal Court, said Matt Smith of Fortify.

"Impunity in Myanmar won't end on its own," he added.