At least 6,700 Rohingya were killed between August and September after violence broke out in Myanmar's northern restive Rakhine state, according to the Medecins Sans Frontieres (MSF).
The figures released on Thursday, which are based on surveys of refugees in Bangladesh, is much higher than Myanmar's official figure of 400.
"We met and spoke with survivors of violence in Myanmar, who are now sheltering in overcrowded and unsanitary camps in Bangladesh," reported CNN, citing Sidney Wong, MSF Medical Director, as saying 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," she said.
The MSF said that it was "the clearest indication yet of the widespread violence" by Myanmar authorities.
The MSF's research, based on the "most conservative estimations", revealed that at least 730 Rohingya children below the age of five were violently killed in the month following August 25.
Earlier, on December 5, the United Nations (UN) Human Rights Chief Zeid Ra'ad Al Hussein condemned the brutal attacks at and decades-long discrimination against the Rohingyas, citing they could contain "elements of genocide".
More than 6,20,000 Rohingyas have fled to Bangladesh since the Myanmar security forces launched an operation in the Rakhine state in response to the alleged attacks by militants on 25 August against 30 police posts and a regimental headquarters.
Earlier on 23 November, Bangladesh and Myanmar signed a deal on repatriation after both the countries came to a consensus following a meeting between Foreign Minister AH Mahmood Ali and Myanmar's State Counsellor Aung San Suu Kyi.
-ANI