Rohingya Muslims in Myanmar are facing a “catastrophic” humanitarian situation, UN Secretary General Antonio Guterres says.
Mr Guterres said alleged attacks by security forces on Rohingya villagers were completely unacceptable, and urged them to suspend military action.
The army says it is fighting militants and denies targeting civilians.
Some 379,000 Rohingyas have fled to Bangladesh since violence began last month. Whole villages have burned down.
The Rohingya, a mostly Muslim minority in the Buddhist-majority Rakhine state, have long experienced persecution in Myanmar, which says they are illegal immigrants. They have lived in Myanmar, also known as Burma, for generations but are denied citizenship.
The UN Security Council is due to meet on Wednesday to discuss the crisis.
But Myanmar officials say the country’s de facto leader, Aung San Suu Kyi, will miss a key debate next week in the UN General Assembly.
She will, however, address the nation on 19 September, the day the General Assembly meets.
Its refugee agency says not enough aid is getting through to the Rohingya who have fled to Bangladesh.
Mr Guterres called on the international community to provide whatever assistance they could.
“The humanitarian situation it is catastrophic,” he said.
“When we met last week there was 125,000 Rohingya refugees who had fled into Bangladesh. That number has now tripled to nearly 380,000.
“Many are staying in makeshift settlements or with those communities who are generously sharing what they have. But women and children are arriving hungry and malnourished.”
Asked whether the crisis could be categorised as ethnic cleansing, Mr Guterres said: “A third of the [Rohingya] population had to flee the country – can you find a better word to describe it?”
The UN secretary general said that he had condemned attacks by the Arakan Rohingya Salvation Army (ARSA), the rebel group fighting the military.
But he said that military action should also be suspended and those who had fled be allowed the right to return home.