A top United Nations envoy is in western Myanmar after an outbreak of religious violence that has left dozens dead.

Vijay Nambiar, UN chief Ban Ki-moon's special adviser on Myanmar, flew into Sittwe, the capital of restive Rakhine state, on Wednesday.

Nambiar will later visit Maungdaw -- on the border with Bangladesh -- where the latest spate of violence began on Friday.

"We're here to observe and assess how we can continue to provide support to Rakhine," Ashok Nigam, UN resident and humanitarian co-ordinator, told AFP news agency.

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.

About 25 people have been killed and a further 41 people were wounded in five days of unrest, an official said.

That death 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 started the violence in Rakhine.

Fleeing to Bangladesh

On Wednesday, New York-based Human Rights Watch urged Bangladesh to keep its border open to people seeking refuge from sectarian violence in western Myanmar.

The group said in a statement that Bangladesh should also allow independent humanitarian agencies free and unfettered access to the border areas.

It expressed its concern after Bangladesh on Tuesday turned away three boats carrying 1,000 Muslims fleeing violence in neighbouring Myanmar.

"By closing its border when violence ... is out of control, Bangladesh is putting lives at grave risk,'' said Bill Frelick, Refugee Program director at Human Rights Watch.

"Bangladesh has an obligation under international law to keep its border open to people fleeing threats to their lives and provide them protection," Frelick said.

It also urged other governments to provide humanitarian assistance and other support for the refugees.