Global humanitarian NGO Medecins Sans Frontieres (MSF) announced on Thursday that at least 6,700 Rohingya were killed after violence broke out in Myanmar's Rakhine state late August.
Based on surveys of refugees in Bangladesh, the number is much higher than Myanmar's official figure of 400, reports the BBC.
"In the most conservative estimations" at least 6,700 of those deaths have been caused by violence, including at least 730 children under the age of five, MSF said in a report.
MSF also known as Doctors Without Borders, said it was "the clearest indication yet of the widespread violence" by Myanmar authorities.
"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," MSF Medical Director Sidney Wong said.
Among the dead children below the age of five, MSF said more than 59 per cent were reportedly shot, 15 per cent burnt to death, 7 per cent beaten to death and 2 per cent killed by landmine blasts.
More than 647,000 Rohingya have fled into Bangladesh after military crackdown began on August 25 following Rohingya Arsa militants attack on more than 30 police posts, the BBC reported.
After an internal investigation, the Myanmar Army in November exonerated itself of any blame regarding the crisis.
It denied killing any civilians, burning their villages, raping women and girls, and stealing possessions.
The Muslim-majority community are denied citizenship by Myanmar, where they are seen as immigrants from Bangladesh. The government does not use the term Rohingya but calls them Bengali Muslims.
"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 added.