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More than 1,000 people may already have been killed in Myanmar, mostly minority Rohingya Muslims - more than twice the government's total - a senior United Nations representative told AFP on Friday.

In the last two weeks alone 270,000 mostly Rohingya civilians have fled to Bangladesh, overwhelming refugee camps that were already bursting at the seams, the UN said.

Others have died trying to flee the fighting in Rakhine state, where witnesses say entire villages have been burned since Rohingya militants launched a series of coordinated attacks on August 25, prompting a military-led crackdown.

On the basis of witness testimonies and the pattern of previous outbreaks of violence, said Yanghee Lee, the UN special rapporteur on human rights in Myanmar, "perhaps about a thousand or more are already dead".

"This might be from both sides but it would be heavily concentrated on the Rohingya population."

The Rohingya have long been subjected to discrimination in mostly Buddhist Myanmar, which regards them as illegal immigrants from Bangladesh and denies them citizenship even if they have lived in the country for generations.

Bangladesh has struggled to cope with the latest influx, which takes the number of Rohingya refugees in camps on its border with Myanmar to around 670,000.

Of these, nearly 357,000 - a third of Myanmar's total Rohingya population - have left since October when the latest upsurge in violence began.

The UN said there was a sharp increase in arrivals on Wednesday, when at least 300 boats from Myanmar landed in Bangladesh.

Scores of Rohingya have drowned trying to make the perilous sea journey in boats that the Bangladesh authorities say are woefully inadequate at this time of year, when the sea is rough. Many of the dead were children.

Lee, a South Korean academic, told AFP she feared "it's going to be one of the worst disasters that the world and Myanmar has seen in recent years".

The figures she gave are far higher than official tolls, which total 432, including 15 security personnel and 30 civilians - seven Rohingyas, seven Hindus and 16 Rakhine Buddhists.

In an interview at Sungkyunkwan University in Seoul, where she is a professor in the department of child psychology and education, Lee said it was "highly possible" the government had "underestimated numbers".

"The unfortunate thing, the serious thing is that we can't verify that now with no access."

Lee expressed scepticism about authorities' claims that the Rohingya were burning their own houses, pointing out that nearby Buddhist villages were untouched - and it is the rainy season.

"If you have got people with guns and you're running away and it's damp, how easily can you set your own house on fire?" she asked.