International aid group Doctors Without Borders said its field survey has found at least 6,700 Rohingya Muslims were killed between August and September in a crackdown by Myanmar's security forces.

The group, known by its French acronym MSF, said in a statement made available Thursday that it conducted the survey in refugee camps in Bangladesh. It estimated at least 9,000 Rohingya had died of various causes in Myanmar's Rakhine state between Aug. 25 and Sept. 24, and that more than 70 per cent of the deaths were the result of violence.

According to MSF, the dead included at least 730 children younger than five.

More than 630,000 Rohingya Muslims have fled Buddhist-majority Myanmar into Bangladesh to escape what the United Nations has called "ethnic cleansing."

"The peak in deaths coincides with the launch of the latest 'clearance operations' by Myanmar security forces in the last week of August," MSF medical director Sidney Wong said in a statement.

She said the findings were staggering, both in terms of the numbers of people who reported a family member dead as a result of violence and the horrific ways in which they said they were killed or severely injured.

MSF said that among children below five years old, more than 59 per cent who were killed during that period were reportedly shot, 15 per cent were burnt to death in their homes, seven per cent were beaten to death and two per cent died due to land mine blasts.

Belief the toll is underestimated

Myanmar's Information Ministry has said 400 people died following attacks by a militant Rohingya group on police posts on Aug. 25. It said most of the 400 were "extremist terrorists" who died during military "clearance operations."

International aid and rights groups have accused the military of arson, killings and rapes of Rohingya villagers. Myanmar authorities have blamed Rohingya militants for the violence.

More than one million ethnic Rohingya Muslims have lived in Myanmar, also known as Burma, for generations. They have been stripped of their citizenship, denied almost all rights and labelled stateless.

Since the Myanmar's military conducted operations against the Rohingya in Rakhine state, the civilian government has barred most journalists, international observers and humanitarian aid workers from independently travelling to the region.

MSF said the number of deaths is likely to be an underestimation "as we have not surveyed all refugee resettlements in Bangladesh and because the surveys don't account for the families who never made it out of Myanmar."