A US government investigation has found that Myanmar's military waged a "well-planned and coordinated" campaign of mass killings, gang rapes and other atrocities against the country's Rohingya Muslim minority.

US officials have said the State Department report could be used to justify further US sanctions or other punitive measures against Myanmar authorities.

The report stopped short of describing the crackdown as genocide or crimes against humanity.

The findings resulted from more than 1,000 interviews of Rohingya men and women in refugee camps in neighbouring Bangladesh, where almost 700,000 Rohingya have fled after a military campaign last year in Myanmar's Rakhine State.

The survey reveals that the recent violence in northern Rakhine state was extreme, large-scale, widespread, and seemingly geared toward both terrorising the population and driving out the Rohingya residents, according to the 20-page report.

It says the scope and scale of the military's operations indicate they were well-planned and coordinated.

Survivors described in harrowing detail what they had witnessed, including soldiers killing infants and small children, the shooting of unarmed men, and victims buried alive or thrown into pits of mass graves.

They described widespread sexual assault by Myanmar's military of Rohingya women, often carried out in public.

One witness described four Rohingya girls who were abducted, tied up with ropes and raped for three days. They were left "half dead," he said, according to the report.

Human rights groups and Rohingya activists have put the death toll in the thousands from the crackdown, which was sparked by attacks by Rohingya insurgents on security forces in Rakhine state in August 2017.

UN report found 'genocidal intent'

The results of the US investigation were being released nearly a month after UN investigators issued their own report accusing Myanmar's military of acting with "genocidal intent" and calling for the country's commander-in-chief and five generals to be prosecuted under international law.

The military in Myanmar, where Buddhism is the main religion, has denied accusations of ethnic cleansing and says its actions were part of a fight against terrorism.

Senior US State Department officials said the objective of the investigation was not to determine genocide but to "document the facts" on the atrocities to guide US policy aimed at holding the perpetrators accountable.

One of the officials, speaking on condition of anonymity, said it would be up to US Secretary of State Mike Pompeo whether to make such a "legal" designation in the future and did not rule out the possibility.

A declaration of genocide by the US government, which has only gone as far as labelling the crackdown "ethnic cleansing," could have legal implications of committing Washington to stronger punitive measures against Myanmar.

The International Criminal Court last week said it had begun an examination of whether the alleged forced deportations of Rohingya could constitute war crimes or crimes against humanity.

Sarah Margon, director of the Washington office of Human Right Watch, said: "What's missing now is a clear indication of whether the US government intends to pursue meaningful accountability and help ensure justice for so many victims."

"The stories from some refugees show a pattern of planning and pre-meditation" by the military and other attackers, the report said, citing confiscation in advance of knives and other tools that could be used as weapons.

About 80% of refugees surveyed said they witnessed a killing, most often by military or police, according to the report.

Reports of mutilation included the cutting and spreading of entrails, severed limbs or hands/feet, pulling out nails or burning beards and genitals to force a confession, or being burned alive, the report said.

In a few cases, witnesses said Rohingya were either killed in pits bulldozed by the military or buried alive.

The Trump administration, which has been criticised by human rights groups and some US politicians for a cautious response to Myanmar, could now face added pressure to take a tougher stand.

The US has announced it is almost doubling its aid for displaced Rohingya Muslims in Bangladesh and Myanmar, with an extra $185 million.

Any stiffer measures against Myanmar authorities could be tempered though by US concerns about complicating relations between civilian leader Aung San Suu Kyi and the powerful military that might push Myanmar closer to China.

The US government on 17 August imposed sanctions on four military and police commanders and two army units but Myanmar's military chief, Min Aung Hlaing, was spared.

The Rohingya, who regard themselves as native to Rakhine state, are widely considered as interlopers by Myanmar's Buddhist majority and are denied citizenship.