The report, based on hundreds of interviews, is the strongest condemnation from the UN so far of violence against the Rohingya.
The army’s tactics are “consistently and grossly disproportionate to actual security threats”, it says.
It names six senior military figures it believes should go on trial.
It is also fiercely critical of Myanmar’s de facto leader, Aung San Suu Kyi, for failing to intervene to stop the violence.
The report calls for the case to be referred to the International Criminal Court (ICC).
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The government has consistently said its operations targeted militant or insurgent threats.
But the report says the crimes it has documented are “shocking for the level of denial, normalcy and impunity that is attached to them”.
“Military necessity would never justify killing indiscriminately, gang raping women, assaulting children, and burning entire villages.”
‘A searing indictment’
Jonathan Head, BBC Southeast Asia correspondent
Genocide is the most serious charge that can be made against a government, and is rarely proposed by UN investigators.
That this report finds sufficient evidence to warrant investigation and prosecution of the senior commanders in the Myanmar armed forces is a searing indictment, which will be impossible for members of the international community to ignore.
However taking Myanmar to the ICC, as recommended by the report, is difficult. It is not a signatory to the Rome Statute that established the court, so a referral to the ICC would need the backing permanent five Security Council members – and China is unlikely to agree.
The report suggests instead the establishment of a special independent body by the UN, as happened with Syria, to conduct an investigation in support of war crimes and genocide prosecutions.
The government of Myanmar has until now rejected numerous investigations alleging massive atrocities by its military. This one will be much harder to dismiss.
What crimes does the UN allege?
The UN’s Independent International Fact-Finding Mission on Myanmar was set up in March 2017 to investigate widespread allegations of human rights abuses in Myanmar, particularly in Rakhine state.
It began before the military started a large scale operation in Rakhine in August 2017, after deadly attacks by Rohingya militants.
At least 700,000 Rohingya fled Myanmar in the subsequent violence.
The situation was a “catastrophe looming for decades” says the report, and the result of “severe, systemic and institutionalised oppression from birth to death”.
The crimes documented in Kachin, Shan and Rakhine include murder, imprisonment, torture, rape, sexual slavery, persecution and enslavement that “undoubtedly amount to the gravest crimes under international law”.
In Rakhine state, the report also found elements of extermination and deportation “similar in nature, gravity and scope to those that have allowed genocide intent to be established in other contexts”.
The UN mission did not have access to Myanmar for its report but says it relied on such sources as eyewitness interviews, satellite imagery, photographs and videos.
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Who does the UN blame?
The UN mission names army officials who it says bear the greatest responsibility. They include Commander-in-Chief Ming Aung Hlaing and his deputy.
The military is described as being virtually above the law.
Under the constitution civilian authorities have little control over the military, but the document says that “through their acts and omissions, the civilian authorities have contributed to the commission of atrocity crimes”.
Nobel Peace Prize laureate Aung Sang Suu Kyi “has not used her de facto position as Head of Government, nor her moral authority, to stem or prevent the unfolding events in Rakhine”.
The report says that some abuses were also committed by armed ethnic groups in Kachin and Shan state, and by the Arakan Rohingya Salvation Army (Arsa) in Rakhine.
The mission said it would release a more detailed report on 18 September.
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What has been happening?
The Rohingya are one of many ethnic minorities in Myanmar and make up the largest percentage of the country’s Muslims. The government, however, sees them as illegal immigrants from neighbouring Bangladesh and denies them citizenship.
The military launched its latest crackdown after militants from Arsa attacked police posts in August 2017, killing several policemen.
The UN has previously described the military offensive in Rakhine as a “textbook example of ethnic cleansing” and refugees who have fled the violence have told horrific stories of sexual violence and torture.
According to the medical charity MSF, at least 6,700 Rohingya, including at least 730 children under the age of five, were killed in the first month after the violence broke out.
The army exonerated itself in an investigation in 2017.
Rights groups like Amnesty International have long called for top officials to be tried for crimes against humanity over the Rohingya crisis.