More than 60 people are presumed dead after a boat carrying Rohingya Muslims fleeing Myanmarinfo-icon capsized, the UN migration agency has said.

"Twenty-three people have been confirmed dead ... 40 are missing and presumed drowned," a spokesman for International Organisation for Migration told reporters in Geneva. "The total fatality toll be in the range of 60," he said, updating a previous toll of 19.

Survivors from the accident on Thursday told IOM staff that the boat was carrying about 80 people, including 50 children, who were believed to be fleeing violence from Myanmar's northern Rakhine state. The boat overturned in rough waters off Bangladeshinfo-icon.

"Survivors described being at sea all night, having no foodinfo-icon," the IOM spokesman added.More than half a million minority Rohingya Muslims have fled an armyinfo-icon campaign in just a few weeks, escaping Myanmar into Bangladesh.

The violence, the latest and most deadly upsurge in years of government oppression and communal hatred between Rohingya and Buddhists in Rakhine, exploded on 25 August when Rohingya insurgents attacked army posts.

A ferocious counteroffensive has destroyed more than 200 Muslim villages, which have been shown by satellite imagery to have been burned. Refugees in Bangladesh have recounted horrific stories of rape, mass murder and infanticide.

Speaking at a damning open session of the United Nationsinfo-icon security council on Thursday night, the UN secretary general Antonio Guterres said the conflictinfo-icon had become "the worldinfo-icon's fastest developing refugee emergency and a humanitarian and human rightsinfo-icon nightmare".

The USinfo-icon ambassador to the UN, Nikki Haley, lambasted the government of Aung San Suu Kyi for the bloodshed. "We cannot be afraid to call the actions of the Burmese authorities what they appear to be: a brutal, sustained campaign to cleanse the country of an ethnic minority," she said. "And it should shame senior Burmese leaders who have sacrificed so much for an open, democratic Burma."

But Myanmar's national security adviser U Thaung Tun denied the accusations. "I can assure you that the leaders of Myanmar, who have been struggling so long for freedom and human rights, will never espouse policy of genocide or ethnic cleansing and that the government will do everything to prevent it," he said.He repeated a government line that 50% of Muslim villages in north Rakhine state, the heart of the violence, remain intact.

U Thaung Tun said Myanmar was "concerned by reports that thousands of people have crossed into Bangladesh" but said the country needed to "fathom the real reasons for the exodus", which he blamed on "terrorists".

But Masud Bin Momen, Bangladesh's representative to the UN, said it was evident why people were escaping. "Any individual among the new arrivals would make it known why this exodus is continuing. They all narrate use of rape as a weapon to scare families to leave," he said.

Myanmar has blocked aid access to the region for United Nations humanitarian agencies, preventing civilians in the conflict zone from receiving food, water and medicine. Meanwhile, aid workers in Bangladesh warn of a humanitarian catastrophe for hundreds of thousands of refugees kept in muddy camps over the border.

The BBC reported on Friday that the head of the UN in Myanmar had been accused of mishandling the long-standing issue by prioritising development in impoverished Rakhine over pushing for Rohingya rights.

It cited Caroline Vandenabeele, former head of office for UN resident coordinator Renata Lok-Dessallien, who said raising the Rohingya problem had negative consequences for UN staff. "An atmosphere was created that talking about these issues was simply not on," she was quoted as saying.

The UN in Myanmar said it strongly disagreed "with the accusations that the resident co-ordinator 'prevented' internal discussions".