UNITED NATIONS - U.N. Secretary-General António Guterres called Thursday for “swift action” to halt the deteriorating situation in Myanmar’s northern Rakhine State, where a half million mostly Rohingya Muslims have fled to neighboring Bangladesh in the past month.

“The situation has spiraled into the world’s fastest developing refugee emergency; a humanitarian and human rights nightmare,” Guterres told an open meeting of the U.N. Security Council.

UN Chief Urges 'Swift Action' to Alleviate Suffering of Myanmar Muslims

The council has privately discussed the situation three times in the past month, but Thursday’s session was the first time since 2009 that it has publicly discussed Myanmar.

UN Chief Urges 'Swift Action' to Alleviate Suffering of Myanmar Muslims

Guterres called for an end to the military’s operations; unhindered aid access; and the safe and voluntary return of refugees to their areas of origin.

UN Chief Urges 'Swift Action' to Alleviate Suffering of Myanmar Muslims

“There seems to be a deeply disturbing pattern to the violence and ensuing large movements of an ethnic group from their homes,” Guterres said.

Guterres and his human rights commissioner have both expressed concerns that what is happening in Rakhine State is ethnic cleansing.

The secretary-general said the core problem is the prolonged statelessness of the Rohingya and its associated discrimination. “The Muslims of Rakhine State should be granted nationality,” Guterres said.

The Rohingya are one of many ethnic minorities in the Buddhist-majority nation. They are considered to be economic migrants from Bangladesh and have been denied citizenship, even though most can show that their families have been in the country for generations.

The Red Crescent Society, which has mounted a $13 million appeal for the Rakhine refugee camps in Cox's Bazar in southern Bangladesh, warns the camps may be on the brink of a possible outbreak of disease.

"Our mobile clinics are treating more people, especially children, who are very sick from diarrheal diseases which are a direct result of the terrible sanitation conditions in the makeshift camps," said Mozharul Huq, Secretary General of the Bangladesh Red Crescent Society. "We are on the cusp of a serious health outbreak."

The Red Crescent says safe water, sanitation, hygiene promotion and medical care are "urgently needed" to prevent what would be a catastrophic development.

"In some camps, hundreds of people are sharing one toilet," said Martin Faller, deputy regional director for Asia Pacific for the International Federation of Red Cross and Red Crescent Societies. "The conditions for an outbreak of disease are all present - we have to act now ."

Violence erupted in Rakhine on August 25, after attacks by Rohingya militants on state security forces led to military reprisals.

U.S. ambassador Nikki Haley said the military response has been “disproportionate and indiscriminate” and has dwarfed the original rebel attacks in the scope of its violence. She called for a suspension of arms sales to the military until sufficient accountability measures are in place.

The military has been accused of the widespread burning of Rohingya villages, rape, killings, looting and the laying of landmines to prevent people returning to their homes.

“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 said, referring to Myanmar by its other name.

“And it should shame senior Burmese leaders who have sacrificed so much for an open, democratic Burma,” she said in an apparent reference to the country’s de facto leader Aung San Suu Kyi.

Nobel Peace Prize laureate Aung San Suu Kyi has faced international criticism for remaining silent on the situation until last week.

Haley said the United States has provided $95 million in humanitarian aid to Myanmar and Bangladesh, but acknowledged it would not be enough for the growing emergency.

“As we speak, the situation on the ground is beginning to move toward stability,” China’s Deputy U.N. envoy Wu Haitao declared.

Russian ambassador Vassily Nebenzia discouraged “excessive pressure” on Myanmar’s authorities, which he said could only aggravate the situation. “We need to be very careful when we wield such notions as genocide and ethnic cleansing,” he added.

Most council members expressed their concern that the crisis could spill over into the region, causing broader instability and the potential radicalization of the disenfranchised. There was also broad consensus among members that the violence must stop, humanitarian agencies must be allowed in, and root causes of the conflict should be addressed.

Myanmar’s national security adviser, Thaung Tun, disputed that ethnic cleansing is taking place.