More than 270,000 have been forced to flee to Bangladesh from Myanmar's Rakhine state
The UN human rights chief says the current situation in Myanmar appears to be "a textbook example of ethnic cleansing".
More than 270,000 people are believed to have fled from Myanmar - also known as Burma - to Bangladesh in recent weeks, amid a humanitarian crisis facing Rohingya Muslims in the country's Rakhine state.
Burmese police and army have launched a violent crackdown in the state following attacks by Rohingya insurgents on police posts last month.
The UN has said it is receiving 'constant reports' of violence carried out by security forces in the country since those attacks, with claims that thousands of people in Rakhine state have been killed or tortured.
There have also been reports that Burmese security forces have begun to place landmines along the border with Bangladesh.
Zeid Ra'ad Al Hussein, the United Nations High Commissioner for Human Rights, addressed the situation in remarks to the UN's Human Rights Council.
He said that the "brutal" security operation is "clearly disproportionate" to last month's attacks, and is "without regard for basic principles of international law".
Mr Zeid observed: "Last year I warned that the pattern of gross violations of the human rights of the Rohingya suggested a widespread or systematic attack against the community, possibly amounting to crimes against humanity, if so established by a court of law.
"Because Myanmar has refused access to human rights investigators the current situation cannot yet be fully assessed, but the situation seems a textbook example of ethnic cleansing."
He called on the Myanmar government to stop claiming the Rohingyas are destroying their own homes & villages, adding that the "complete denial of reality" is damaging a government that previously enjoyed "immense" international good will.
He added: "I call on the Government to end its current cruel military operation, with accountability for all violations that have occurred and to reverse the pattern of severe and widespread discrimination against the Rohingya population."
Myanmar's de facto leader Aung San Suu Kyi has faced mounting international criticism over her response to the crisis in her country, including from her fellow Nobel Peace Prize winner Desmond Tutu.