A top UN envoy arrived in strife-torn western Myanmar on Wednesday as security forces grappled with sectarian violence that has left dozens dead and hundreds of homes burned down.

A state of emergency has been declared in Rakhine state, which has been rocked by a wave of rioting and arson, posing a major test for the reformist government which took power last year. A dusk-to-dawn curfew has been imposed in many areas.

Vijay Nambiar, UN chief Ban Ki-moon's special adviser on Myanmar, flew into the capital of Rakhine to visit Maungdaw, a town near the border with Bangladesh where the violence between Buddhists and Muslim Rohingya flared on Friday.

He was accompanied by Myanmar's Border Affairs Minister General Thein Htay and 15 Muslim religious leaders from Yangon.

"We're here to observe and assess how we can continue to provide support to Rakhine," Ashok Nigam, UN resident and humanitarian coordinator who was also in the group, told AFP.

Gunfire has rattled Sittwe in recent days as plumes of smoke rise from fires blazing around the city. Local residents have been seen roaming the streets wielding knives, swords and sticks.

The Buddhists and Rohingya have both accused each other of violent attacks.

The UN has evacuated most of its foreign staff from Maungdaw, which is its main base in the state and has a large population of stateless Rohingya Muslims.

Around 25 people have been killed and a further 41 people were wounded in five days of unrest, an official told AFP on Tuesday. He did not give details of how they died or whether they were Buddhists or Muslims.

Rohingya leaders say the real number of dead is much higher but AFP could not verify the allegation and has been unable to visit many of the affected areas for security reasons.

The toll does not include 10 Muslims who were killed on June 3 by a Buddhist mob in apparent revenge for the rape and murder of a woman, which sparked the violence in Rakhine.

Hundreds of Rohingya, many of them women and children, have attempted to flee to Bangladesh in rickety boats but have been turned away by border guards.

Rakhine, a predominantly Buddhist state bordering Bangladesh, is home to a large number of Muslims including the Rohingya, described by the United Nations as one of the world's most persecuted minorities.

The Myanmar government considers the Rohingya to be foreigners, while many citizens see them as illegal immigrants and view them with hostility, describing them as "Bengalis".

The UN Refugee Agency (UNHCR) has urged Bangladesh to let the fleeing Rohingya in.

"On these boats are women and children, and injured people," UNHCR representative Craig Sanders told AFP in Dhaka.

"We are appealing to the Bangladesh government to keep open its border and provide emergency and other humanitarian assistance."

The United States has urged an immediate halt to the deadly sectarian unrest while New York-based Human Rights Watch has called for international observers to be deployed, warning that the violence is running "out of control".

A spokesman for opposition leader Aung San Suu Kyi's National League for Democracy party said the former political prisoner had instructed him to work "to help both sides equally" before she left Wednesday on a historic trip to Europe.

The veteran activist, who will formally accept her 1991 Nobel Peace Prize in Oslo on Saturday, has remained largely silent on the unrest apart from calling for "sympathy for minorities", while key figures in the democratic movement have come out to say the Rohingya are not one of Myanmar's ethnic nationalities.