United Nations Secretary General Antonio Guterres is urging Myanmar's authorities to immediately end military operations that have sent half a million Rohingya Muslims fleeing to Bangladesh.

Guterres told the UN Security Council Thursday that the violence had spiralled into the "world's fastest developing refugee emergency, a humanitarian and human rights nightmare." 

"We have received bone-chilling accounts from those who fled — mainly women, children and the elderly," he said. "These testimonials point to excessive violence and serious violations of human rights, including indiscriminate firing of weapons, the use of landmines against civilians, and sexual violence." 

Guterres said the humanitarian crisis is a breeding ground for radicalization, criminals and traffickers.

He told the Security Council that Myanmar must also allow "unfettered access" for humanitarian aid and ensure the return of all those who sought refuge in Bangladesh. 

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Rohingya Muslim refugees in Kutupalong refugee camp in Ukhia on Wednesday, the same day the UN Security Council met to discuss the humanitarian crisis. (Fred Dufour/AFP/Getty Images)

​It's estimated more than 500,000 Rohingya Muslims have fled in the past month since insurgents attacked security posts near the border, triggering fierce Myanmar military retaliation that the United Nations has previously branded ethnic cleansing.

Three boats carrying refugees capsized Thursday, killing 15 people, including several children, the UN International Organization for Migration said.

Breeding ground for 'radicalization'

"The failure to address this systematic violence could result in a spillover into central Rakhine, where an additional 250,000 Muslims could potentially face displacement," Guterres said. 

"The crisis has generated multiple implications for neighbouring states and the larger region, including the risk of inter-communal strife. We should not be surprised if decades of discrimination and double standards in treatment of the Rohingya create openings for radicalization," he said. 

U.S. Ambassador to the UN Nikki Haley called on countries to suspend providing weapons to Myanmar until the military puts sufficient accountability measures in place — something human rights groups have been urging.

"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," Haley told the UN Security Council.

Human Rights Watch and 86 other non-governmental organizations, including several Canadian groups, urged the UN to take immediate action.  

"As more evidence emerges, it is clear that the atrocities committed by Myanmar state security forces amount to crimes against humanity," the coalition said in a statement

It's calling for the UN General Assembly to consider an arms embargo against the Myanmar military and targeted sanctions against individuals responsible for crimes and serious abuses. 

The organizations said it was not enough for UN members to "hold meetings and make speeches as atrocities continue."

International aid groups in Myanmar released a joint statement calling for free access to Rakhine State, where they said their work has been hampered by the Myanmar government.

An unknown number of people are internally displaced, while hundreds of thousands lack food, shelter and medical services, said the groups, including Care International, Oxfam and Save the Children.  

They said they were "increasingly concerned about severe restrictions on humanitarian access and impediments to the delivery of critically needed humanitarian assistance throughout Rakhine State."

Guterres invited to visit

Myanmar rejects accusations of ethnic cleansing and crimes against humanity, but leader Aung San Suu Kyi has faced scathing criticism and calls for her Nobel Prize to be withdrawn. 

In an address last week, she denounced rights abuses and expressed concern about the suffering. She also said any refugees verified as coming from Myanmar would be allowed to return.

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Nobi Hossain wades through the water carrying his elderly relative Sona Banu as hundreds of Rohingya refugees arrive under the cover of darkness from Myanmar to the shore of Shah Porir Dwip, Bangladesh, on Wednesday. (Damir Sagolj/Reuters)

Myanmar national security adviser Thaung Tun said at the UN Thursday there was no ethnic cleansing or genocide happening in Myanmar. He told the Security Council that Myanmar had invited Guterres to visit. A UN official said Guterres would consider visiting Myanmar under the right conditions. 

China and Russia both expressed support for the Myanmar government. 

Thaung said the crisis in Rakhine state "is due to terrorism and is not based on religion," and urged the Security Council not to take measures that would exacerbate the situation.​