UN chief Antonio Guterres exhorted Myanmar's leaders to end the "nightmare" faced by Rohingya refugees fleeing an army campaign, after at least 19 people drowned with scores more feared dead when a boat carrying Rohingya families capsized off Bangladesh.
More than half a million Rohingya Muslims have poured into Bangladesh in the last month, fleeing a vicious Myanmar military crackdown on Rohingya rebels that has gutted villages across northern Rakhine state.
Scores have drowned while trying to cross waters separating the two countries, while those who survive face new dangers as they cram into squalid refugee settlements where food and clean water are in short supply.
The billowing humanitarian crisis prompted the UN Security Council to hold its first meeting on Myanmar in eight years, though the member countries failed to arrive at a joint resolution.
The US slammed the army for trying "to cleanse the country of an ethnic minority", while Beijing and Moscow offered support to Myanmar authorities who have vehemently rebuffed allegations that ethnic cleansing is underway.
Speaking to the 15-member council, Guterres urged Myanmar to halt military operations and open humanitarian access to the conflict-wracked western region.
"The situation has spiralled into the world's fastest developing refugee emergency, a humanitarian and human rights nightmare," he said, while calling for those displaced from the conflict to be allowed to return home.
The UN chief noted that the "systemic violence" could cause unrest to spill south to the central part of Myanmar's Rakhine state, threatening 250,000 Muslims with displacement.
Some of the strongest criticism came from US envoy Nikki Haley, who accused Myanmar authorities of waging a "brutal, sustained campaign to cleanse the country of an ethnic minority".
"It should shame senior Burmese leaders who have sacrificed so much for an open, democratic Burma," she added, in what appeared to be a rebuke to the country's civilian leader Aung San Suu Kyi, whose reputation as a human rights champion has been battered by the crisis.
Burma is an alternative name for Myanmar.
But Myanmar received strong backing from Russia and China, a close ally and key trade partner.
"The international community must be aware of the difficulties faced by the Burmese government, be patient and provide its assistance," Chinese envoy Wu Haitao said.
"We must be very careful when we talk about ethnic cleansing and genocide," added Russian ambassador Vassily Nebenzia, taking the Myanmar government line as he blamed Rohingya militants for "burning villages".