Myanmar's de facto leader Aung San Suu Kyi has "a last chance" to halt an army offensive that has forced hundreds of thousands of the mainly Muslim Rohingyas to flee abroad, the UN head has said.
Antonio Guterres told the BBC that unless she acted now, "the tragedy will be absolutely horrible".
The UN has warned the offensive could amount to ethnic cleansing.
Myanmar says it is responding to last month's deadly attacks by militants and denies it is targeting civilians.
The military launched its operation after the attacks on police in the northern Rakhine state.
In an interview with BBC's HARDtalk programme ahead of this week's UN General Assembly, Mr Guterres said Aung San Suu Kyi had a last chance to stop the offensive during her address to the nation on Tuesday.
"If she does not reverse the situation now, then I think the tragedy will be absolutely horrible, and unfortunately then I don't see how this can be reversed in the future."
The secretary-general reiterated that the Rohingyas should be allowed to return home.
He also said it was clear that Myanmar's military "still have the upper hand" in the country, putting pressure "to do what is being done on the ground" in Rakhine.
Aung San Suu Kyi - a Nobel Peace Prize laureate who spent many years under house arrest in the junta-run Myanmar (Burma) - is now facing growing criticism over the Rohingya issue.
She will not be attending the UN General Assembly in New York, and has claimed that the crisis is being distorted by a "huge iceberg of misinformation".
She said tensions were being fanned by fake news promoting the interests of terrorists.
Mr Guterres' warning comes after Bangladesh said it was now limiting the movement of more than 400,000 Rohingyas who have fled from Myanmar.
WHY IS AID SLOW TO ARRIVE?
Bangladeshi police said Rohingyas 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.
Bangladesh also announced plans to build shelters for up to 400,000 people near the city of Cox's Bazar.
Analysts say the government wants to stop the Rohingya from disappearing into the general population and to keep them visible, in the hope of returning them to Myanmar - or even a third country.
On 25 August, Rohingya militants attacked police posts in northern Rakhine, killing 12 security personnel.
Rohingya who have fled Myanmar since then say the military responded with a brutal campaign, burning villages and attacking civilians in a bid to drive them out.
The Rohingya, a stateless mostly Muslim minority in Buddhist-majority Rakhine, have long experienced persecution in Myanmar, which says they are illegal immigrants.
Some who fled from Rakhine state told the BBC earlier this month about killings, rape and even massacres, while inside Rakhine, a BBC crew witnessed charred homes inside Rakhine.
A new Human Rights Watch report released on Friday accused the Myanmar military of an "ethnic cleansing campaign" and detailed scores of villages targeted with arson attacks.
Myanmar officials blame the Rohingya insurgents for the violence, with government spokesman Zaw Htay urging displaced people to find refuge in temporary camps set up in Rakhine state.
However, Mr Htay said Myanmar would not be able to allow all those who fled to Bangladesh to return.