Updated

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 had conducted the survey in refugee camps in Bangladesh and estimated that at least 9,000 Rohingya died in Myanmar's Rakhine state between Aug. 25 and Sept. 24. About 630,000 Rohingya have fled Myanmar into Bangladesh to escape what the United Nations has called "ethnic cleansing."

The estimate of the number of deaths announced by the group compares to Myanmar's government figure of 400 in September as a result of attacks on police posts by Rohingya militants.

According to MSF, the dead included at least 730 children younger than 5.

Myanmar's Information Ministry had said that most of the 400 dead were "extremist terrorists" who died during the military's "clearance operations." International aid and rights groups have accused the military of arson, killings and rapes of Rohingya villagers. Myanmar authorities blamed Rohingya militants for the violence.

Though more than 1 million ethnic Rohingya Muslims have lived in the country for generations, they were stripped of their citizenship, denied almost all rights and labeled stateless.

"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 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 horrific ways in which they said they were killed or severely injured.

MSF said that among children below the age of 5, more than 59 percent who were killed during that period were reportedly shot, 15 percent burnt to death in their homes, 7 percent beaten to death and 2 percent died due to land mine blasts.

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

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