Myanmar's army chief should be prosecuted for "genocide" against the Rohingya, a United Nations' human rights investigator has said.

Yanghee Lee, the UN special rapporteur on human rights in Myanmar, added that holding the perpetrators to account for their crimes was necessary before the refugees who fled the country could return.

Lee, who is barred from Myanmar, was speaking on Friday during a trip to Thailand and Bangladesh, where she met officials and Rohingya driven out of western Rakhine state after an army crackdown in 2017.

"Min Aung Hlaing and others should be held accountable for genocide in Rakhine and for crimes against humanity and war crimes in other parts of Myanmar," said Lee, referring to the military's commander-in-chief.

It was the first time Lee had publicly called for the Myanmar army chief to be prosecuted for genocide.

Min Aung Hlaing and others should be held accountable for genocide in Rakhine Yanghee Lee, UN rights envoy

'Genocidal intent'

A UN fact-finding mission on Myanmar last year said that the military campaign, which refugees say included mass killings and rape, was orchestrated with "genocidal intent" and recommended charging Min Aung Hlaing and five other generals with the "gravest crimes under international law".

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Since August 2017, some 750,000 Rohingya have fled Rakhine to Bangladesh, where they now live in overcrowded camps.

"For any repatriation to happen ... the perpetrators must be held to account because sending the refugees back with no accountability is going to really exacerbate or prolong the horrific situation in Myanmar," Lee told Reuters news agency.

"And then we'll see another cycle of expulsion again."

Spokespersons for Myanmar's military and government could not be reached for comment.

The country has previously denied almost all allegations made by refugees against its troops, which it says were engaged in legitimate counterterrorism operations.

On Thursday, Lee visited an island off Bangladesh where Dhaka aims to move Rohingya refugees despite worries it will be vulnerable to extreme weather.

Bangladesh is spending $280m transforming Bhashan Char, a muddy silt islet that only emerged from the sea two decades ago, into a camp for some of the refugees.

But the island, in a coastal region where weather has killed hundreds of thousands of people in recent decades, is one hour by boat from the nearest land over a stretch of sea prone to violent storms.

Some of the Rohingya themselves, living in overcrowded and squalid camps in southeastern Bangladesh border district of Cox's Bazar, have expressed unease about moving, while the UN has insisted that any relocation must be voluntary.

Global call for action

The UN Security Council in September voted to approve the establishment of an "ongoing independent mechanism" for Myanmar that would collect, consolidate, and preserve evidence of crimes that could be used in an eventual court case.

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Myanmar has said it "absolutely rejects" that the International Criminal Court (ICC) has jurisdiction to rule on its actions. The country is not a party to the Rome Statute that established the Hague-based court.

Non-parties can be referred to the ICC by the UN Security Council, though diplomats have said permanent members China and Russia would likely veto any such move.

Legal experts say other options for an international prosecution include referral by individual UN member states - five Latin American states recently successfully referred Venezuela - or an ad hoc tribunal.