Since renewed violence erupted in Buddhist-majority Myanmar last month, around 300,000 Muslim Rohingya have fled across the Naf river into Bangladesh.
But the United Nations High Commissioner for Human Rights, Zeid Ra'ad Al Hussein, says it's been an extremely concerning situation for some time.
"Last year I warned that the pattern of gross violations of the human rights of the Rohingyas suggested a widespread or systematic attack against the community, possibly amounting to crimes against humanity, if so established by a court of law. Because Myanmar has refused access to human rights investigators, the current situation cannot yet be fully assessed, but the situation remains, or seems, a textbook example of ethnic cleansing."
Vivian Tan is from the office of the UN High Commissioner for Refugees (UNHCR), and is currently in Bangladesh.
"These people are very weak. They are mainly women and children and some elderly. Many say they have walked for days to get to Bangladesh, their homes are burnt in Myanmar and they had to flee. Some of them described fleeing through the mountain, hiding in the jungle and one of them said nine days, it took them nine days to get to Bangladesh."
The refugees are being housed in makeshift camps in and around the city of Cox's Bazar, which is near the Myanmar border.
It's left aid agencies struggling to deal with what they are calling a growing humanitarian crisis.
Vivian Tan says they are working with authorities to try and provide life-saving assistance.
"This is not ideal. We are trying to find more land together with the government to create new emergency shelter and outside the two camps there are many many people who are living under makeshift shelters that have just sprouted up on the side of the roads."
The UNHCR has launched a global campaign raise almost US$7.5 million to help it deliver life-saving aid.
Last month, the military launched a counter-offensive following attacks by the Arakan Rohingya Salvation Army insurgents on police posts and an army base.
Myanmar's military says its forces are fighting a legitimate campaign against "terrorists", who they blame for the attacks on them and for burning homes and civilian deaths.
Zeid Ra'ad Al Hussein has told the UN Human Rights Council in Geneva it's been a disproportionate response without regard for basic principles of international law.
"We have received multiple reports and satellite imagery of security forces and local militia burning Rohingya villages and consistent accounts of extrajudicial killings, including shooting fleeing civilians. I am further appalled by reports that the Myanmar authorities have now begun to lay landmines along the border with Bangladesh."
Myanmar's elected leader, Aung San Suu Kyi, is under mounting international pressure to halt the violence.
Critics complain that Ms Suu Kyi, who won the Nobel peace prize for championing democracy, has failed to speak out for a minority that has long complained of persecution.
The UN's Rights Commissioner is urging authorities to allow the UN unfettered access to the country.
"I call on the government to end its current cruel military operation with accountability for all violations that have occurred and to reverse the pattern of severe and widespread discrimination against the Rohingya population."
