The top United Nations human rights official said Myanmar's leader Aung San Suu Kyi promised on Friday to investigate U.N. allegations of atrocities against Rohingya Muslims.

Security forces and police have committed mass killings and gang rapes and burned villages in northern Rakhine state, a U.N. investigation published on Friday found.

"I did speak to Aung San Suu Kyi about an hour and a half ago. I called upon her to use every means available to exert pressure on the military and the security services to end this operation," U.N. High Commissioner for Human Rights Zeid Ra'ad al-Hussein said in an interview with Reuters in Geneva.

"She informed me that an investigation will be launched. She said that they would require further information."

In Yangon, presidential spokesman Zaw Htay said: "These are extremely serious allegations, and we are deeply concerned. We will be immediately investigating these allegations through the investigation commission led by Vice-President U Myint Swe.

"Where there is clear evidence of abuses and violations, we will take all necessary action."

Myanmar, a mostly Buddhist country, has previously denied almost all allegations of human rights abuses against Muslims in northern Rakhine and says a lawful counterinsurgency campaign is under way.

Since it began on Oct. 9, about 69,000 Rohingya have fled to neighbouring Bangladesh. The U.N. report was based on accounts gathered in January from 220 of them.

Witnesses testified to "the killing of babies, toddlers, children, women and elderly; opening fire at people fleeing; burning of entire villages; massive detention; massive and systematic rape and sexual violence; deliberate destruction of food and sources of food".

One woman described her baby's throat being slit. Another was raped by soldiers and saw her five-year-old daughter killed.

The report said the actions by security forces probably amounted to crimes against humanity.

"HORRORS"

Zeid said the perpetrators of such "horrors" must be held to account. Possible avenues would be the establishment of an international commission of inquiry or the involvement of the International Criminal Court.

The report described "area clearance operations" - gunfire and grenades dropped on villages from helicopters - which probably killed hundreds.

Nearly half of those interviewed said a family member had been killed or disappeared while 101 women reported having been raped or subjected to sexual violence.

Testimonies pointed to "a persecution on ethnic grounds which is similar to what has been, in other contexts, described as 'ethnic cleansing'," U.N. mission leader Linnea Arvidsson told a news briefing.

The investigators took evidence including photographs of bullet and knife wounds, burns, and injuries resulting from beatings with rifle butts or bamboo sticks.

The plight of the stateless Rohingya, of whom some 1.1 million live in apartheid-like conditions in Rakhine, has long been a source of friction between Myanmar and Bangladesh.

Many Rohingya had hoped that Suu Kyi, a Nobel Peace Prize winner, would work to restore their rights once her civilian administration took power in March last year.

But within weeks of the latest crisis erupting, diplomats and aid workers were privatedly expressing dismay at her lack of deeper involvement.

"I am not going to go now into the extent to which she should have done more or less," Zeid said. "There has to be some responsibility."

Officials have so far denied observers and independent journalists access to the conflict area, while accusing Rohingya of fabricating stories and collaborating with insurgents who they say are terrorists with links to Islamists overseas.