YANGON (Reuters) - Myanmar said on Friday a visiting U.S. official would not be allowed to go to a region where violence has triggered an exodus of nearly 400,000 Rohingya Muslims that the United Nations has branded a “textbook example of ethnic cleansing”.

The Rohingya have fled from western Rakhine state to neighboring Bangladesh to escape a military offensive that has raised questions about Myanmar’s transition to civilian rule under the leadership of Nobel laureate Aung San Suu Kyi.

U.S. Deputy Assistant Secretary of State Patrick Murphy will voice Washington’s concerns about the Rohingya and press for greater access to the conflict area for humanitarian workers, the State Department said.

Myanmar officials said he would meet government leaders in the capital, Naypyitaw, and attend an address to the nation by Suu Kyi on Tuesday.

He would also visit Sittwe, the state capital, and meet the governor of Rakhine, the state government secretary, Tin Maung Swe, told Reuters, but the north of the state, where the conflict erupted on Aug. 25 would be off limits.

“Not allowed,” Tin Maung Swe said, when asked if Murphy would be going to Maungdaw district, at the heart of the strife that began when Rohingya insurgents attacked police posts and an army camp, killing a dozen people.

While nearly 400,000 refugees have poured across the border into Bangladesh, fears have also been growing of a humanitarian crisis on the Myanmar side, but access for aid workers and reporters has been severely restricted.

Myanmar insisted on Friday it was not barring aid workers but a government spokesman said authorities on the ground might have concerns over security.

Rights monitors and fleeing Rohingya say the army and Rakhine Buddhist vigilantes have mounted a campaign of arson aimed at driving out the Muslim population.

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A Reuters photographer on the Bangladesh side of the border said he could see huge banks of dark smoke billowing up over Myanmar territory on Friday, while international aid organizations said the refugees kept coming.

“There’s really no sign that this flow of people is going to dry up,” Chris Lom of the International Organisation for Migration, said from the Bangladeshi border district of Cox’s Bazar.

“There are still, we believe, thousands of people waiting to take boats across to Cox’s Bazar.”

‘SCORCHED EARTH’

U.N. Secretary-General Antonio Guterres and the U.N. Security Council have urged Myanmar to end the violence, which he said was best described as ethnic cleansing.

Myanmar rejects the accusations, saying its security forces are carrying out clearance operations to defend against the insurgents of the Arakan Rohingya Salvation Army, which claimed responsibility for the Aug. 25 attacks and similar, though smaller, attacks in October.

Russia’s U.N. Ambassador Vassily Nebenzia said on Friday that while Moscow was concerned “we cannot discard the fact that it was triggered by terrorist activities.”

“We’re talking to the Myanmar government, of course,” he told reporters. “We are concerned with what is happening. ... We are looking forward to an early cessation of the violence and to elevating the humanitarian situation of the population.”

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The government has declared Arakan Rohingya Salvation Army a terrorist organization and accused it of setting the fires and attacking civilians.

Ethnic cleansing is not recognized as a separate crime under international law but allegations of ethnic cleansing as part of wider, systematic human rights violations have been heard in international courts.

Rights group Amnesty International said evidence pointed to a “mass-scale scorched-earth campaign” across the north of Rakhine that was unmistakably ethnic cleansing.

“The evidence is irrefutable – the Myanmar security forces are setting northern Rakhine state ablaze in a targeted campaign to push the Rohingya people out of Myanmar,” said Tirana Hassan, the group’s crisis response director.

The group said it had detected 80 big fires in Rohingya areas since Aug. 25. While the extent of damage could not be verified, due to access restrictions by the government, “they are likely to have burned down whole villages”.

It said it also had credible reports of Rohingya militants burning the homes of ethnic Rakhine and other minorities. About 30,000 non-Muslims have also been displaced.

U.S. Senate Majority Leader Mitch McConnell said on Thursday he had spoken with Suu Kyi and that she said she was working to get aid to areas in Myanmar affected by violence.

The generals still control national security policy but nevertheless, Suu Kyi has been widely criticized abroad for not stopping or condemning the violence. The campaign against the Muslim insurgents is popular at home.

The U.N. refugee agency said the Rohingya arriving in Bangladesh were suffering “real hardship, and some of the most difficult conditions seen in any current refugee situation”.

“With the influx increasing daily, UNHCR is appealing for an initial amount of $30 million for the emergency humanitarian response in Bangladesh until the end of year,” an agency spokesman said in Geneva.

Bangladesh says all refugees must go home and has called for safe areas in Myanmar. Myanmar has ruled that out and says it will accept anyone who can verify their citizenship. Most Rohingya are stateless.

Thousands of people demonstrated after Friday prayers in the Bangladeshi capital, Dhaka, to denounce the treatment of the Rohingya.

One protest leader said they were demanding a U.N. peacekeeping force and that Myanmar face charges in an international court.