Burma said on Friday a visiting US 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 Arakan State to neighbouring Bangladesh to escape a military offensive that has raised questions about Burma’s transition to civilian rule under the leadership of Nobel laureate Aung San Suu Kyi.

US 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.

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

He would also visit Sittwe, the Arakan State capital, and meet the chief minister of Arakan, the state government secretary, Tin Maung Swe, told Reuters, but the north of the state, where the conflict erupted on 25 August, 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 Burma side, but access for aid workers and reporters has been severely restricted.

Burma insisted on Friday that 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 Arakanese Buddhist vigilantes have mounted a campaign of arson aimed at driving out the Muslim population.

A Reuters photographer on the Bangladesh side of the border said he could see huge banks of dark smoke billowing up over Burmese territory on Friday, while international aid organisations 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.”

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

Burma 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 25 August attacks and similar, though smaller, attacks in October.

The government has declared it a terrorist organisation and accused it of setting the fires and attacking civilians.

Ethnic cleansing is not recognised 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 Arakan State that was unmistakably ethnic cleansing.

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

The group said it had detected 80 big fires in Rohingya areas since 25 August. 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 Arakanese and other minorities. About 30,000 non-Muslims have also been displaced.

US 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 Burma affected by violence.

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

The UN 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.”

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“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 Burma. Burma 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 UN peace-keeping force and that Burma face charges in an international court.