The UN Security Council will meet Thursday to discuss the violence in Myanmar and hear a briefing from UN Secretary-General Antonio Guterres on the crisis, diplomats said.
Britain, France, the United States and four other countries requested the meeting after people, mostly Rohingya Muslims, fled an army campaign in Myanmar in recent weeks and crossed into Bangladesh.
The United Nations said on Tuesday about 480,000 Rohingya Muslims had fled to Bangladesh since violence broke out in Myanmar on August 25, increasing its estimate by 45,000 in two days.
A report by UN agencies and international charities said the change was due largely to an estimated 35,000 Rohingya, who had not previously been accounted for, moving into two refugee camps.
It also said numbers crossing the border had started to rise again. The UN gave a figure of more than 435,000 Rohingya arrivals on Saturday.
The United Nations has described the military operation as "ethnic cleansing" and French President Emmanuel Macron last week went further, calling it "genocide."
Turkish President Recep Tayyip Erdogan, who has denounced the plight of the Rohingya, accused Myanmar of waging a "Buddhist terror" against the Muslim minority and also denounced the "genocide."
The meeting was also requested by Egypt, Kazakhstan, Senegal and Sweden, which are non-permanent council members.
Council members were set to get an update on the situation in Myanmar on Tuesday, ahead of the formal briefing by the UN chief, which will be held in an open session.
In early September, Guterres took the rare step of sending a letter to the council to express concern about the "humanitarian catastrophe" unfolding, raising fears that it could have "implications for peace and security" beyond Myanmar's borders.
A Reuters report adds from Yangon, the Human Rights Watch said on Tuesday Myanmar is committing crimes against humanity in its campaign against Muslim insurgents in Rakhine state. It called for the UN Security Council to impose sanctions and an arms embargo.
A government spokesman rejected the accusation, saying there was no evidence, adding that the government was committed to protecting rights.
Myanmar has also rejected UN accusations that its forces are engaged in ethnic cleansing against Rohingya Muslims in response to coordinated attacks by Rohingya insurgents on the security forces on Aug. 25.
It says its forces are fighting terrorists responsible for attacking the police and the army, killing civilians and torching villages.
They have accused the security forces and Buddhist vigilantes of trying to drive Rohingya out of Buddhist-majority Myanmar.
"The Burmese military is brutally expelling the Rohingya from northern Rakhine State," said James Ross, legal and policy director at New York-based Human Rights Watch.
"The massacres of villagers and mass arson driving people from their homes are all crimes against humanity."
The International Criminal Court defines crimes against humanity as acts including murder, torture, rape and deportation "when committed as part of a widespread or systematic attack directed against any civilian population, with knowledge of the attack".
Human Rights Watch said its research, supported by satellite imagery, had found crimes of deportation and forced population transfers, murder and attempted murder, rape and persecution.
The UN Security Council and concerned countries should impose targeted sanctions and an arms embargo, the group said.
Government spokesman Zaw Htay said no Myanmar government had ever been as committed to the promotion of rights as the current one.
"Accusations without any strong evidence are dangerous," he told Reuters. "It makes it difficult for the government to handle things."
The violence and the refugee exodus represent the biggest crisis the government of Nobel peace laureate Aung San Suu Kyi has faced since it came to power last year in a transition from nearly 50 years of harsh military rule.
Myanmar regards the Rohingya as illegal immigrants from Bangladesh and bouts of suppression and violence have flared for decades. Most Rohingya are stateless.
The United States and its Western allies imposed sanctions on Myanmar for years in support of Suu Kyi's campaign for democracy.
Seven UN experts, including Yanghee Lee, special rapporteur on rights in Myanmar, called on Suu Kyi to meet Rohingya to hear for herself the reasons for their exodus.
"No one chooses, especially not in the hundreds of thousands, to leave their homes and ancestral land, no matter how poor the conditions, to flee to a strange land to live under plastic sheets and in dire circumstances, except in life-threatening situations," they said.
They called on Myanmar to provide humanitarian access to Rakhine state, where the military has been restricting entry.
Suu Kyi has little, if any, control over the security forces under a military-drafted constitution that also bars her from the presidency and gives the military veto power over political reform.
Myanmar has seen a surge of Buddhist nationalism in recent years, and the public is supportive of the campaign against the insurgents.
Since Sunday, the army has unearthed the bodies of 45 members of Myanmar's small Hindu community who authorities say were killed by the insurgents soon after the violence erupted.
The Arakan Rohingya Salvation Army, which has claimed attacks on the security forces since October, denied killing the villagers.
Some Hindus have fled to Bangladesh. Others have taken refuge in Myanmar towns, accusing the insurgents of attacking them on suspicion of being government spies.
Another report by AP adds, Myanmar's UN ambassador insisted Monday that there is no "ethnic cleansing" or genocide taking place against Muslims and objected "in the strongest terms" to countries that used those words to describe the situation in Rakhine State.
Hau Do Suan used his "right of reply" at the end of the six-day gathering of world leaders at the General Assembly to respond to what he called "irresponsible remarks" and "unsubstantiated allegations" repeated by countries in their speeches to the 193-member world body.
He didn't identify any of the nations, though many spoke out about the plight of the fleeing Rohingya Muslims.
Among those who accused Myanmar of trying to rid itself of Rohingya were Bangladesh Prime Minister Sheikh Hasina, UN Secretary-General Antonio Guterres, UN human rights chief Zeid Ra'ad al-Hussein, and a number of Islamic countries, including the United Arab Emirates.
Suan denied those claims.
"There is no ethnic cleansing. There is no genocide," he said. "The leaders of Myanmar, who have long been striving for freedom and human rights, will not espouse such policies. We will do everything to prevent ethnic cleansing and genocide."
He called the issue of Rakhine State "extremely complex" and urged UN member states and the international community "to see the situation in northern Rakhine objectively and in an unbiased manner."
