* Violence comes amid reports of vigilantes burning villages
* Around 27,400 Rohingya cross into Bangladesh since Friday
* Around 20,000 Rohingya stranded in no man's land
* U.S. calls on Myanmar to protect civilians (Adds U.N. expert, paragraphs 10 & 11)
By Nurul Islam
COX'S BAZAR, Bangladesh, Aug 31 (Reuters) - Bangladeshi border guards have recovered two dozen bodies from the country's shore in the last two days, as tens of thousands of Rohingya Muslims make desperate attempts to flee the worst violence involving the Myanmar minority in at least five years.
At the United Nations, the U.S. ambassador to the world body, Nikki Haley, urged Myanmar's security forces to avoid attacking innocent civilians.
Haley condemned recent attacks by the Arakan Rohingya Salvation Army but added "as Burmese security forces act to prevent further violence, they have a responsibility to adhere to international humanitarian law, which includes refraining from attacking innocent civilians and humanitarian workers."
She also urged security forces to ensure aid reaches those in need and to ensure the rights of all communities.
Around 27,400 Rohingya Muslims have crossed into Bangladesh from Myanmar since Friday, three U.N. sources said, after Rohingya insurgents wielding sticks, knives and crude bombs attacked police posts and an army base in Rakhine state, leading to clashes that killed at least 117 people.
Myanmar said its security forces were carrying out clearance operations in northern Rakhine to defend against "extremist terrorists". Monitors said fleeing Rohingya reported that the army and ethnic Rakhine Buddhist vigilantes have unleashed a+ campaign of arson aimed at driving out the Muslim population.
Reuters reporters in Bangladesh on Thursday saw a huge fire on the Myanmar side of the Naf River.
Many villages were also on fire near the town of Maungdaw in Rakhine, where another Reuters reporter saw charred debris and smoke billowing from the forest.
The U.N. sources in Bangladesh said around 20,000 Rohingya were stranded in no man's land between the two countries. One predicted the figure could jump to 30,000 later on Thursday.
Yanghee Lee, the U.N. special rapporteur on human rights in Myanmar, said the humanitarian situation was deteriorating rapidly.
"Many thousands of people are increasingly at risk of grave violations of their human rights," she said in a statement. "The worsening cycle of violence ... must be broken urgently."
Myanmar has evacuated thousands of Buddhists from Rakhine since the start of the fighting that has mainly killed Rohingya insurgents but also security force personnel, according to the Myanmar government.
The treatment of about 1.1 million Rohingya in Myanmar is the biggest challenge facing Myanmar leader Aung San Suu Kyi, accused by Western critics of not speaking out for a minority that has long complained of persecution.
On Thursday, the bodies of 11 Rohingya children and nine women washed up on the Bangladesh side of the Naf after their boat overturned, said Ariful Islam, a Bangladesh border guard commander.
The bodies of two Rohingya women and two children were recovered on Wednesday after their boat was fired on by Myanmar's Border Guard Police, Islam said. A Rohingya leader in Bangladesh cited survivors saying both boats were overcrowded.
In the Bangladeshi border district of Cox's Bazar, makeshift camps for the displaced set up since similar violence last October were being expanded.
One arrival, Mohammed Rashid, 45, wore a surgical dressing under his eye, saying bullet splinters injured him after the Myanmar army opened fire on a group of Rohingya.
He said about 100 people made their way to the border together, and he saw explosions and people dying.
"We hid in the forest for two days and then we were stopped at the border, but we got through. We heard that the houses in our village have burned down," Rashid told Reuters.
Chris Lewa of the Arakan Project, a Rohingya monitoring group, said it appeared Myanmar security forces were trying to drive out much of the Rohingya population. She said ethnic Rakhine vigilantes were "participating in the burning of villages".
"What we're hearing is burning, burning, burning," she said. "And it seems to be spreading from south to north."
Myanmar has said it has the right to defend itself from attack, adding that security personnel were told to protect innocent civilians.
The Myanmar army has said it is battling insurgents who continued to ambush government forces. Monitors said there have been few, if any, insurgent attacks reported since Friday's initial offensive.
The violence marks a dramatic escalation of a conflict that has simmered since October, when a similar but much smaller series of Rohingya attacks on security posts prompted a brutal military response dogged by allegations of rights abuses.
The Rohingya are denied citizenship in Myanmar and regarded as illegal immigrants, despite claiming roots that date back centuries. Bangladesh is also growing increasingly hostile to Rohingya, more than 400,000 of whom live in the poor South Asian country after fleeing Myanmar since the early 1990s.
Bangladesh on Wednesday pushed back 366 Rohingya trying to enter the country though thousands have set up temporary camps along the porous land border, borders guards said.
The International Organization for Migration joined U.N. Secretary-General Antonio Guterres in appealing to Bangladesh to admit people caught on the border. Bangladesh has insisted it lacks resources to care for them.
Aid workers say contingency stocks of materials are low and their biggest problem is extreme overcrowding in makeshift settlements.
(Reporting by Nurul Islam, Ruma Paul, Reuters Staff; additional reporting by Michelle Nichols at the United Nations in New York and Stephanie Nebehay in Geneva; Writing by Krishna N. Das and David Brunnstrom; Editing by Alex Richardson and Jonathan Oatis)
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