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.
WHERE IS AUNG SAN SUU KYI?
Nobel laureate Aung San Suu Kyi is in a delicate position. She is Myanmar's de facto leader, but security is under the control of the autonomous armed forces.
If Ms Suu Kyi bows to international pressure and sets up a credible investigation into the alleged abuses in Rakhine state, she risks fracturing her relationship with the army. It could jeopardise the stability of her young government.
So for the last six weeks Ms Suu Kyi has kept her head firmly in the sand, avoiding journalists and press conferences.
When forced, she has commented that the military in Rakhine is operating according to the "rule of law". Few believe that to be the case.
While there are loud calls from overseas for action, most Burmese have very little sympathy for the Rohingya. The army's "clearance operations" against the "violent attackers" of Rakhine state appear to have strong popular support, putting Ms Suu Kyi under very little domestic pressure.
Myanmar's presidential spokesman Zaw Htay said Mr McKissick "should maintain his professionalism and his ethics as a United Nations officer because his comments are just allegations".
"He should only speak based on concrete and strong evidence on the ground," he said.
On Wednesday, the Bangladesh foreign ministry summoned Myanmar's ambassador to express "deep concern" over the military operation in northern Rakhine state.
It said "desperate people" were crossing the border seeking safety and shelter and asked Myanmar to "ensure the integrity of its border".
Authorities in Bangladesh have been detaining and repatriating hundreds of fleeing Rohingya, which Amnesty International condemned as a violation of international law.
Bangladesh does not recognise Rohingya as refugees, and many of those fleeing Myanmar have been "forced into hiding and are suffering a severe lack of food and medical care", the rights group said.
Rohingya refugees and asylum-seekers have arrived into Bangladesh from Myanmar in waves since at least the 1970s. There are some 33,000 registered Rohingya refugees living in Cox's Bazar's two camps, Kutupalong and Nayapara.
Earlier this week, Human Rights Watch released satellite images which it said showed that more than 1,200 homes had been razed in Rohingya villages over the past six weeks.
WHAT IS HAPPENING IN RAKHINE STATE?
A massive security operation was launched last month after nine police officers were killed in co-ordinated attacks on border posts in Maungdaw.
Some government officials blamed a militant Rohingya group for the attacks. Security forces then sealed off access to Maungdaw district and launched a counter-insurgency operation.
Rohingya activists say more than 100 people have been killed and hundreds arrested amid the crackdown.
Soldiers have also been accused of serious human rights abuses, including torture, rape and executions, which the government has flatly denied.
It says militants have attacked helicopter gunships providing air support to troops.
WHO ARE THE ROHINGYA?
The estimated one million Muslim Rohingya are seen by many in mainly Buddhist Myanmar as illegal migrants from Bangladesh. They are denied citizenship by the government despite many having lived there for generations.
Communal violence in Rakhine state in 2012 left scores dead and displaced more than 100,000 people, with many Rohingya still remaining in decrepit camps.
They face widespread discrimination and mistreatment.
Hundreds of thousands of undocumented Rohingya are estimated to live in Bangladesh, having left Myanmar over decades.
IS THE GOVERNMENT TO BLAME?
Myanmar held its first openly contested election in 25 years last November, with Nobel Laureate Aung San Suu Kyi's National League for Democracy winning a landslide victory.
Though she is barred from the presidency due to a constitutional rule, Ms Suu Kyi, who serves as State Counsellor, is seen as de-facto leader.
But her government, led as it is by a former human rights icon, has faced international criticism over the dire situation in Rakhine state.
Rights groups have questioned why journalists and aid workers are not being allowed to enter northern Rakhine.
Presidential spokesman Zaw Htay says the international media is misreporting what is going on, according to BBC News.