Myanmar is seeking the ethnic cleansing of the Muslim Rohingya minority from its territory, a senior UN official has told the BBC.
Armed forces have been killing Rohingya in Rakhine state, forcing many to flee to neighbouring Bangladesh, says John McKissick of the UN refugee agency.
The government of Myanmar, also known as Burma, has been conducting counter-insurgency operations since coordinated attacks on border guards in October.
It denies reports of atrocities.
A spokesman said the government was "very, very disappointed" by the comments.
Burmese officials say Rohingya are setting fire to their own houses in northern Rakhine state. The BBC cannot visit the area to verify what is occurring there, as journalists and aid workers have been barred.
The Rohingya, who number about one million, are seen by many of Myanmar's Buddhist majority as illegal migrants from Bangladesh.
Although Bangladesh's official policy is not to allow in illegal entrants across the border, the foreign ministry has confirmed that thousands of Rohingya have already sought refuge in the country, while thousands more are reportedly gathering on the border.
Some are using smugglers to get into Bangladesh, while others have bribed border guards, according to Amnesty International.
Efforts to resolve the issue must focus on "the root cause" inside Myanmar, Mr McKissick, head of the UN refugee agency UNHCR in the Bangladeshi border town of Cox's Bazar, told BBC Bengali's Akbar Hossain.
He said the Myanmar military and Border Guard Police had "engaged in collective punishment of the Rohingya minority" after the murders of nine border guards on 9 October which some politicians blamed on a Rohingya militant group.
Security forces have been "killing men, shooting them, slaughtering children, raping women, burning and looting houses, forcing these people to cross the river" into Bangladesh, Mr McKissick said.
"Now it's very difficult for the Bangladeshi government to say the border is open because this would further encourage the government of Myanmar to continue the atrocities and push them out until they have achieved their ultimate goal of ethnic cleansing of the Muslim minority in Myanmar," he said.