Myanmar's security forces may be guilty of genocide against the Rohingya Muslim minority and more of them are fleeing despite a deal between Myanmar and Bangladesh to send them home, the top UN human rights official said on Tuesday.

The United Nations defines genocide as acts meant to destroy a national, ethnic, racial or religious group in whole or in part. Such a designation is rare under international law, but has been used in contexts including Bosnia, Sudan and an Islamic State campaign against the Yazidi communities in Iraq and Syria.

Zeid Ra'ad al-Hussein, UN High Commissioner for Human Rights, said that none of the 626,000 Rohingya who have fled violence to Bangladesh since August should be repatriated to Myanmar unless there was robust monitoring on the ground.

Myanmar's ambassador, Htin Lynn, said his government was working with Bangladesh to ensure returns of the displaced in about two months and "there will be no camps".

Zeid, who has described the campaign in the past as a "textbook case of ethnic cleansing", was addressing a special session of the UN Human Rights Council called by Bangladesh.

He described reports of "acts of appalling barbarity committed against the Rohingya, including deliberately burning people to death inside their homes, murders of children and adults; indiscriminate shooting of fleeing civilians; widespread rapes of women and girls, and the burning and destruction of houses, schools, markets and mosques".

"Can anyone - can anyone - rule out that elements of genocide may be present?" he told the 47-member state forum.

Shahriar Alam, Bangladesh's junior foreign affairs minister, told the session in Geneva that his country was hosting nearly one million "Myanmar nationals" following summary executions and rapes "as a weapon of persecution".

Mainly Buddhist Myanmar denies the Muslim Rohingya are its citizens and considers them foreigners.

These crimes had been "perpetrated by Myanmar security forces and extremist Buddhist vigilantes", Alam said, calling for an end to what he called "xenophobic rhetoric..including from higher echelons of the government and the military".

Zeid urged the Council to recommend that the UN General Assembly establish a new mechanism "to assist individual criminal investigations of those responsible".

Prosecutions for the violence and rapes against Rohingya by security forces and civilians "appear extremely rare", he said.

Marzuki Darusman, head of an independent international fact-finding mission on Myanmar, said by video from Malaysia: "We will go where the evidence leads us."

His team has interviewed Rohingya refugees, including children in the Bangladeshi port city of Cox's Bazar, who recounted "acts of extreme brutality" and "displayed signs of severe trauma".

Myanmar has not granted the investigators access to Rakhine, the northern state from which the Rohingya have fled, Darusman said. "We maintain hope that it will be granted early in 2018."

Shahriar Alam, Bangladesh's junior foreign affairs minister, told the session in Geneva that his country was hosting nearly one million "Myanmar nationals" following summary executions and rapes "as a weapon of persecution".

Mainly Buddhist Myanmar denies the Muslim Rohingya are its citizens and considers them foreigners.

These crimes had been "perpetrated by Myanmar security forces and extremist Buddhist vigilantes", Alam said, calling for an end to what he called "xenophobic rhetoric..including from higher echelons of the government and the military".

These crimes had been "perpetrated by Myanmar security forces and extremist Buddhist vigilantes

Pramila Patten, special envoy of the UN Secretary-General on sexual violence in conflict, who interviewed survivors in Bangladesh in November, said: "I heard the most heart-breaking and horrific accounts of sexual atrocities reportedly committed in cold blood out of a lethal hatred of these people solely on the basis of their ethnicity and religion".

Crimes included "rape, gang rape by multiple soldiers, forced public nudity and humiliation, and sexual slavery in military captivity", Patten said.

Myanmar denies committing atrocities against the Rohingya. Its envoy Htin, referring to the accounts, said: "People will say what they wanted to believe and sometimes they will say what they were told to say."

Kelley Currie, US ambassador to the UN Economic and Social Council, said the Rohingya's lack of Myanmar citizenship was "the fundamental root cause of this crisis", adding: "Stop denying the seriousness of the current situation."

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