UN secretary-general Antonio Guterres has said that ethnic cleansing is taking place in Burma against the Rohingya Muslim minority.
His comments came as the UN Security Council condemned the violence that has led nearly 380,000 Rohingyas to flee to Bangladesh.
Britain's UN Ambassador Matthew Rycroft said the council's press statement, following closed-door consultations, was the first statement the UN's most powerful body has made in nine years on the situation in Burma. He called it "an important first step".
The secretary-general has been outspoken about calling for an end to the violence in Burma's Rakhine state, and sent the Security Council an official letter raising his concerns about a crisis - the first time a UN chief has done so since 1989.
The Security Council has not agreed on a Burma statement since 2008 when the country was in a transition to democracy, mainly because of neighbouring China's support for the government and its military.
Human Rights Watch and Amnesty International had accused the Security Council on Tuesday of ignoring large-scale "ethnic cleansing" of the Rohingyas and demanded that it hold an open meeting and urge an end to the violence.
"It's a first step, but the council needs to escalate this to the level that a crisis that bears the hallmarks of ethnic cleansing deserves," Human Rights Watch's UN Director Louis Charbonneau said after the council statement. "We haven't seen that yet."
Mr Guterres at a wide-ranging press conference called Burma one of two world issues "at the top of global concerns", along with North Korea.
"Grievances that have been left to fester for decades have now escalated beyond Myanmar's borders, destabilising the region," the secretary-general warned. "The humanitarian situation is catastrophic."
Last week there were 125,000 Rohingya refugees who had fled to Bangladesh and "the number has now tripled to nearly 380,000", he said.
Mr Guterres was asked whether he agreed with UN human rights chief Zeid Ra'ad al-Hussein that what is happening in Rakhine State is ethnic cleansing. He answered with a question: "When one-third of the Rohingya population had to flee the country, can you find a better word to describe it?"
Rohingya have faced decades of discrimination and persecution by the majority Buddhist population in Burma, where they are denied citizenship despite centuries-old roots in the country.
The current crisis erupted on August 25, when an insurgent Rohingya group attacked police outposts in Burma's Rakhine state, killing a dozen security personnel.
That prompted Myanmar's military to launch "clearance operations" against the rebels, setting off a wave of violence that has left hundreds dead, thousands of homes burned, and tens of thousands fleeing to Bangladesh.
Mr Guterres reiterated his condemnations of the attacks by the insurgent Arakan Rohingya Salvation Army but said reported attacks by security forces against civilians "are completely unacceptable".
The secretary-general called on Burma's authorities "to suspend military action, end the violence, uphold the rule of law, and recognise the right of return of all those who had to leave the country." He also reiterated his call for the government to grant the Rohingyas nationality or at least legal status so they can get jobs, education and health care.