UN chief Antonio Guterres says Myanmar leader Aung San Suu Kyi has one "last chance" to stop the country's refugee crisis.
Over 400,000 Rohinyga Muslims have fled western Rakhine state into Bangladesh since a fresh outbreak of violence at the end of August, according to UN figures.
"I would expect that the leader of the country would be able to contain it, and would be able to reverse the situation," the UN secretary general told the British broadcaster BBC.
"She has a chance, she has a last chance, in my opinion, to do so."
Acknowledging that the army - which still has large sway over politics in Myanmar after decades of military rule - also had a role to play, Guterres said that "the tragedy will be absolutely horrible," if the situation cannot be reversed.
The Rohingya crisis is set to figure as a key issue at the UN General Assembly in New York, where world leaders are gathering for meetings and to make speeches over the coming week.
Suu Kyi is skipping an appearance at the meeting, and is to instead make a televised address in Myanmar on Tuesday to talk about the crisis.
The exodus from western Myanmar began on August 25, when the military launched a crackdown in Rakhine after suspected Rohingya militants attacked army bases and police posts, leaving 12 security officials dead. The army said it had killed nearly 400 suspected insurgents.
Guterres' warning comes after Bangladesh said it was limiting the movement of more than 400,000 Rohingya who have fled from Myanmar.
Bangladeshi police said Rohingya would not be allowed to travel anywhere outside of their allocated homes, not even to live with family or friends.
Transport operators and drivers have also been urged not to carry refugees, with landlords told not to rent out any property to them.