The top United Nations envoy to Myanmar says the country's Rohingya Muslims must have a path to citizenship, otherwise their security will remain under threat.

Vijay Nambiar, in a speech to the International Peace Institute on Thursday, also said that if such a step were not taken "it would affect the international reputation of the country".

Myanmar sees Rohingya Muslims, who live in Rakhine state, as illegal immigrants from Bangladesh even though many were born to families who arrived in the country generations ago.

The million-plus population does not officially exist on government records and the government has said the community is barred from a national census unless its members identify themselves as Bengali.

A report from a human rights group published in February claimed that state policies restricted the Rohingya's "movement, marriage, childbirth, home repairs and construction of houses of worship".

Earlier this month, another UN envoy said severe shortages of food, water and medical care for Rohingya Muslims in western Myanmar were part of a long history of persecution against the religious minority that could amount to "crimes against humanity".

Tomás Ojea Quintana's statement followed the evacuation of hundreds of international humanitarian workers from Rakhine.

Myanmar has also faced criticism from US President Barack Obama, who last month said the country could move in a "very bad direction" if people were politically organised around their religious or ethnic identity.

The sultan of Brunei has announced a controversial new penal code based on Islamic criminal punishments criticised by UN human rights officials.

"Today... I place my faith in and am grateful to God the almighty to announce that tomorrow, Thursday May 1, will see the enforcement of sharia law phase one, to be followed by the other phases," Sultan Hassanal Bolkiah said.

Islamic law penalties would be introduced over time and would eventually include flogging, amputation and death by stoning for various crimes.

Many members of the Muslim ethnic Malay majority have voiced cautious support for the changes. However, non-Muslim citizens led a rare burst of criticism on social media earlier this year, but largely went silent after the sultan called for a halt.

"Theory states that God's law is harsh and unfair, but God himself has said that his law is indeed fair," the sultan said.

The UN's human rights office recently said it was "deeply concerned", adding that penalties such as stoning were classified under international law as "torture or other cruel, inhuman or degrading treatment or punishment".

Brunei officials have said such cases would require an extremely high burden of proof and judges would have wide discretion to avoid such punishments.

The sultan has warned of pernicious foreign influences such as the internet, and indicated he intends to place more emphasis on Islam.

Nearly 70 percent of Brunei's 400,000 people are Muslim Malays while about 15 percent are non-Muslim ethnic Chinese.