At least 6,700 Rohingya Muslims were killed in the first month of a Myanmar army crackdown in Rakhine state that began in late August, Doctors Without Borders (MSF) said.

The figure is the highest estimated death toll yet of violence since a militant attack on police posts on 25 August sparked a deadly crackdown by the Myanmar military and triggered a massive refugee crisis with more than 620,000 refugees crossing into Bangladesh.

The UN and US have described the military operation as ethnic cleansing of the Muslim minority, but have not released specific estimations of a death toll.

"At least 6,700 Rohingya, in the most conservative estimations, are estimated to have been killed, including at least 730 children below the age of five," MSF said.

The group's findings come from six surveys of more than 2,434 households in Rohingya refugee camps and cover a period of one month.

"We met and spoke with survivors of violence in Myanmar, who are now sheltering in overcrowded and unsanitary camps in Bangladesh," said the group's medical director Sidney Wong.

"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."

Gunshot wounds were the cause of death in 69% of the cases, according to the survey.

Another 9% were reported burned alive inside houses, while 5% died from fatal beatings.

For children under five, nearly 60% died after being shot, the survey found.

Myanmar's army has denied any abuses and says only 400 people - including 376 Rohingya "terrorists" - died in the first few weeks of the crackdown.

But MSF said the peak in deaths coincided with the launch of "clearance operations" by the army and local militias in late August, and were evidence "that Rohingya have been targeted".

Director of MSF in Ireland Sam Taylor said children were being killed in a "brutal fashion" and sexual violence was a major problem.

He said around 120,000 Rohingya Muslims remained in northern Rahkine state and MSF was "calling for access to these communities, as we are extremely concerned for those who are stuck there".

Speaking on RTÉ's Morning Ireland, he said MSF clinics in Myanmar have been burned down and they have no independent access to assess the health of these people.

"Some small agencies are helping them but operations have been massively reduced since the crackdown in August