The United Nations has received daily reports of rapes and killings of the Rohingya minority in Burma.
UN human rights chief Zeid Ra'ad Al Hussein said the government, led by Nobel Peace Prize winner Aung San Suu Kyi, had taken a "short-sighted, counterproductive, even callous" approach to the crisis.
He said the government's handling of issues in northern Rakhine state, where independent monitors are barred from investigating, risk grave long term repercussions for the region.
At least 86 people have been killed, according to state media, and the UN estimates 27,000 members of the largely stateless Rohingya minority have fled across the border from Rakhine into Bangladesh.
The High Commissioner said killings, rapes and the burning of Rohingya homes are reported to the UN human rights office on a daily basis.
"The repeated dismissal of the claims of serious human rights violations as fabrications, coupled with the failure to allow our independent monitors access to the worst affected areas in northern Rakhine, is highly insulting to the victims and an abdication of the government's obligations under international human rights law," Mr Zeid said in a statement.
"If the authorities have nothing to hide, then why is there such reluctance to grant us access? Given the continued failure to grant us access, we can only fear the worst."
UN human rights spokeswoman Ravina Shamdasani said the UN human rights office had submitted a formal request for access to the area, which had not yet been granted.
Adrian Edwards, a spokesman for the UN refugee agency UNHCR, said his colleagues in Bangladesh had spoken to more than 1,000 newly-arrived refugees in the past few weeks who gave accounts of houses being burned, targeting of civilians and traumatised women and children who had witnessed the killing of family members.
UNHCR could not verify the accounts first-hand, but it was extremely concerned and it urged the Myanmar authorities to investigate and the government of Bangladesh to give the refugees a safe haven, he said.
Burmese government officials have denied allegations of abuse and said the army is hunting "terrorists" behind raids on police in October.
Mr Zeid said in June that crimes against humanity may have been committed against the Rohingya.
Ms Shamdasani said if the government did not handle the situation very carefully and address the grievances of the Rohingya minority, violence could ensue.
"Unfortunately this is exactly what has happened in the past couple of months," she added. "We are worried that this is going to get further out of hand. This is perfect breeding ground for violent extremists."
Although they have lived in Burma for generations, Rohingya Muslims are barred from citizenship in the nation of 50 million, and instead live as some of the most oppressed people in the world.
Since communal violence broke out in 2012, more than 120,000 Rohingya have been driven from their homes and crammed into squalid camps guarded by police. There, they are denied healthcare and education, and their movements are heavily restricted.
Some have tried to flee by boat, but many ended up becoming victims of human trafficking or were held for ransom.
Additional reporting by Reuters
Join our new commenting forum
Join thought-provoking conversations, follow other Independent readers and see their replies