"They killed the baby by stomping on it with heavy boots. Then they burned the house," a 19-year-old woman witness told United Nations investigators.

Five soldiers were taking turns to rape a 25-year-old woman after they had butchered her husband with a knife, when her eight-month-old son started crying because he was hungry and wanted to be breast-fed.

"To silence him they killed him too with a knife," the 19-year-old testified.

A five-year-old girl ran screaming to try to protect her mother as she was being gang raped, when one of the rapists pulled out a long knife and slit the child's throat.

"I thought I would die but I survived," the mother told investigators.

A just-released UN report based on the testimonies of Rohingya who have reached refugee camps in Bangladesh details mass gang rape, killings, brutal beatings, the torching of homes and people and disappearances in what the UN says "very likely" amounts to crimes against humanity.

The report describes evidence gathered during interviews with more than 200 Rohingya in the camps as a "calculated policy of terror" by Myanmar's security forces under the guise of a military lockdown of Muslim villages.

It states that for decades almost one million Rohingya in Rakhine have suffered systemic discrimination and policies of exclusion and marginalisation.

For months Myanmar's government has denied almost all allegations of human rights abuses in Rakhine, claiming a lawful counter-insurgency operation is underway after attacks on police posts in early October.

But the report crushes expectations that Myanmar's decades of military-led repression had come to an end when a pro-democracy movement led by Nobel laureate Aung San Suu Kyi won a landslide election in late 2015.

The report's release in Geneva comes at a time of heightened religious and communal tensions in the Buddhist-majority country following the assassination last week of Ko Ni, a prominent Muslim lawyer and key member of the ruling National League for Democracy, who was drafting democratic reforms to Myanmar's military-authored Constitution.

And security analysts fear the treatment of the Rohingya will lead to a new long-term insurgency emerging in South-east Asia, backed by international Islamic extremists.

Zeid bin Raad al-Hussein, the UN's High Commissioner for Human Rights, has called for the perpetrators of what he called the Rakhine "horrors" to be held to account, possibly through the establishment of an international commission of inquiry or the involvement of the International Criminal Court.

"The devastating cruelty to which these Rohingya children have been subjected is unbearable," he said. "What kind of hatred could make a man stab a baby crying out for his mother's milk?"

Prince Zeid said Ms Suu Kyi, who has been widely criticised for failing to stand up for Rohingya, promised to investigate after he telephoned her on Saturday (Australian time) and urged her to use every means available to exert pressure on the military and security forces to end their operations in Rakhine.

"She said they would require further information," he said.

The military still wields enormous power in Myanmar and Ms Suu Kyi's relationship with the generals who control key security ministries remains fragile.

In Yangon, presidential spokesman Zaw Htay said "these are extremely serious allegations, and we are deeply concerned".

He said they would be probed by an already established investigation commission which in January downplayed reports of human rights abuses in Rakhine, despite the fact that more than 60,000 Rohingya had fled their homes since October, and the publication of numerous detailed reports of military killings and abuse in the state, including by Amnesty International and Human Rights Watch, which obtained satellite images of the widespread destruction of Rohingya villages.

UN investigators gathered the evidence in the camps after Myanmar's government refused them unfettered access to the worst-affected areas of Rakhine.

They obtained images and videos of bullet and knife wounds, burns, and other serious injuries from people they said were deeply traumatised.

Victims told how hundreds of Rohingya houses, schools, markets, shops and mosques were burned by police, the army and sometimes civilian mobs.

They described the destruction of food and food sources, including paddy fields, and the confiscation of livestock.

"Testimonies were collected of several cases where the army or Rakhine villagers locked an entire family, including elderly and disabled people, inside a house and set it on fire, killing them all," the report said.

The report said the violence raises serious concerns that Myanmar forces are engaged in "ethnic cleansing" to force Rohingya from Rakhine, where they have lived for generations.

Many Burmese ultra-nationalists claim the Rohingya are illegal immigrant "Bengalis" with no cultural, religious or social ties to Myanmar.

Rohingya are routinely denied freedom of movement and other basic rights, including citizenship, in the country.

More than 600 Rohingya have been arrested in the latest operations, and some are believed to have been released, but no details have been released of their fate.

Intermittent violent clashes between Buddhist nationalists and Rohingya have erupted in Rakhine since 2012, forcing tens of thousands of Rohingya into squalid camps.

The report quotes one survivor as saying "now is the worst it has ever been … we have heard from our grandparents that there were bad things happening in the past, but never like this".