At least 6,700 Rohingya died as a result of violence in Rakhine state between 25 August and 24 September, according to a new report based on interviews with refugees.
More than half a million Rohingya have fled northwestern Myanmar into neighboring Bangladesh since a concerted military crackdown began in August, following militant attacks on a border post. Both the UN and the UK have described the campaign as ethnic cleansing.
M-decins Sans Fronti-res (MSF) said at least 9,000 Rohingya died during that period, with more than 71% of the reported deaths caused by violence.
"We met and spoke with survivors of violence in Myanmar, who are now sheltering in overcrowded and unsanitary camps in Bangladesh," Sidney Wong, MSF Medical Director, said in a statement.
"What we uncovered was staggering, both in terms of the numbers of people who reported a family member died as a result of violence and the horrific ways in which they said they were killed or severely injured."
He added the "peak in deaths coincides with the launch of the latest 'clearance operations' by Myanmar security forces in the last week of August."
Zaw Htay, a Myanmar government spokesman, did not respond to a request for comment on the MSF report.
Stories of horror
In an exclusive CNN report published last month, refugees described surviving mass killings and rape before reaching the relative safety of the camps, where conditions are dire.
"They killed and killed and piled the bodies up high. It was like cut bamboo," said Mumtaz, a Rohingya woman from the village of Tula Toli in western Myanmar, who woke up to find herself on a mound of charred bodies.
"In the pile there was someone's neck, someone's head, someone's leg. I was able to come out, I don't know how."
A report by commissioned by Myanmar's military into the crisis cleared the army of wrongdoing, denying widespread reports of murder, rape and destruction in Rakhine state.
Amnesty International described the report as an attempt by the military to "sweep serious violations against the Rohingya under the carpet."
"There is overwhelming evidence that the military has murdered and raped Rohingya and burned their villages to the ground," Amnesty said.
The Myanmar military report attributed the mass exodus of refugees and the repeated reports of military violence to a campaign of misinformation perpetrated by the Rohingya militant group Arakan Rohingya Salvation Army (ASRA).
'Likely an underestimation'
According to the new MSF report, "between 9,425 and 13,759 Rohingya died during the initial 31 days following the start of the violence, including at least 1,000 children below the age of 5 years."
Gunshots were cause of death in 69% of the violence-related killings, followed by being burned to death in hours (9%) and beaten to death (5%). Of the children under 5 who died during that period, 2% were killed by landmines.
"The numbers of deaths are likely to be an underestimation as we have not surveyed all refugee settlements in Bangladesh and because the surveys don't account for the families who never made it out of Myanmar," Wong said.
"We heard reports of entire families who perished after they were locked inside their homes, while they were set alight."