GENEVA - Myanmar's security forces have committed mass killings and gang rapes of Rohingya Muslims and burned their villages since October in a campaign that "very likely" amounts to crimes against humanity and possibly ethnic cleansing, the UN human rights office said on Friday.Witnesses had 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", the report said.One woman told UN investigators how her eight-month baby boy had had his throat slit. Another was raped by soldiers and saw her five-year-old daughter killed as she tried to stop them."The devastating cruelty to which these Rohingya children have been subjected is unbearable," UN High Commissioner for Human Rights Zeid Ra'ad al Hussein said in a statement."The 'area clearance operations' have likely resulted in hundreds of deaths," the UN report said.Around 66,000 people have fled from the Muslim-majority northern part of Rakhine State to Bangladesh since Myanmar's military launched a security operation in response to attacks on police border posts on Oct. 9, the UN report said. The UN humanitarian office has recently put the figure at 69,000.The UN report was issued in Geneva after the investigators gathered testimony last month from 220 Rohingya victims and witnesses who fled the "lockdown area" in Maungdaw in Rakhine for the Cox's Bazar district in Bangladesh.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.Myanmar has denied almost all allegations of human rights abuses in northern Rakhine and says a lawful counterinsurgency campaign is under way.While denying observers and independent journalists access to the conflict area, officials have accused Rohingya residents and refugees of fabricating stories of killings, beatings, mass rape and arson in collaboration with insurgents who they say are Rohingya terrorists with links to Islamists overseas.Zeid called for a robust reaction from the international community and said Myanmar must accept responsibility for committing grave human rights violations against its own people."What kind of hatred could make a man stab a baby crying out for his mother's milk? And for the mother to witness this murder while she is being gang-raped by the very security forces who should be protecting her - what kind of 'clearance operation' is this?" he asked."What national security goals could possibly be served by this?"Bangladesh is determined to relocate Rohingya Muslims fleeing violence in Myanmar to an island in the Bay of Bengal, a Bangladeshi minister said on Wednesday. Critics say the island is uninhabitable. The minister said the move was temporary and Myanmar would ultimately have to take the Rohingyas back.