UN chief Antonio Guterres has acknowledged that Rohingya Muslims were being ethnically cleansed in Myanmar as he called for a halt to a military campaign in Rakhine state.
Speaking ahead of a Security Council meeting to discuss the worsening humanitarian crisis there, Mr Guterres described reports of security forces attacking civilians in Rakhine as "disturbing" and "completely unacceptable."
"I call on the Myanmar authorities to suspend military activities and violence and uphold the rule of law," the secretary general said in a press conference.
Asked if he agreed the Rohingya population was being ethnically cleansed, he replied: "When one-third of the Rohingya population has got to flee the country, can you find a better word to describe it?"
The crackdown by Myanmar's army, which followed attacks by Rohingya militants late last month, has prompted around 380,000 to flee across the border into neighboring Bangladesh.
Mr Guterres called for authorities "to assure the delivery of vital humanitarian aid," with thousands of Rohingya still crossing the border every day.
The 1.1-million strong Rohingya have suffered years of discrimination in Myanmar, where they are denied citizenship even though many have longstanding roots in the country.
But Mr Guterres said that the Myanmar government should either grant the Rohingya nationality or legal status that would allow them to live a normal life.
Myanmar's leader Aung San Suu Kyi has been widely condemned for a lack of moral leadership and compassion in the face of the crisis, denting the Nobel peace laureate's reputation.
However Ms Suu Kyi, Myanmar's first civilian leader in decades, has no control over the powerful military, which ran the country for 50 years before elections in 2015.
She will not attend the upcoming UN General Assembly session in New York.
Critics have called for her to be stripped of her Nobel Peace Prize for failing to do more to halt the strife.
Five women who won the Nobel peace prize have told Ms Suu Kyi she has a personal and moral responsibility to defend the Rohingya people in Burma.
In a letter the laureates accuse the de-facto Burmese leader of indifference over the plight of the Muslim minority.
"As a fellow Nobel Laureate, a worldwide icon for the universal freedom and human rights, and now State Counsellor and de-facto Prime Minister of Burma, you have a personal and moral responsibility to uphold and defend the rights of your citizens," the Nobel Women's Initiative wrote.
"How many Rohingya have to die; how many Rohingya women will be raped; how many communities will be razed before you raise your voice in defence of those who have no voice? Your silence is not in line with the vision of 'democracy' for your country that you outlined to us, and for which we all supported you over the years."
Five Nobel laureates signed the letter earlier this week - Mairead Maguire, from Northern Ireland; Jody Williams from the US; Shirin Ebadi from Iran; Leymah Gbowee from Liberia; and Tawakkol Karman from Yemen.
Ms Maguire said the Nobel Women's Initiative had been in contact with Ms Suu Kyi since her release in 2010 and raised concerns about the Rohingya people with her in person at the UN last year.
"We have been trying really to do something and speak on their behalf," Ms Maguire said.
"We will continue to do that. We just hope out of her conscience she will realise that she has to speak and that this is ethnic cleansing that is going on."
In the letter the laureates praised Ms Suu Kyi for her "struggle for democracy, defiant activism and unimaginable and inspiring sacrifices".
But they called on her to take a firm stand on the crisis and recognise the Rohingya people as Burmese citizens with full rights.
They added: "In the words of fellow Laureate Archbishop Desmond Tutu: 'If the political price of your ascension to the highest office in Myanmar is your silence, the price is surely too steep.' The time is now for you to stand for the rights of Rohingya people, with the same vigour and conviction so many around the world stood for yours."