"For us, it was clear... that these operations were organised and planned," UN High Commissioner for Human Rights Zeid Ra'ad Al Hussein said in an interview.
"You couldn't exclude the possibility of acts of genocide... You cannot rule it out as having taken place or taking place."
Doctors Without Borders said Thursday that at least 6,700 Rohingya were killed in the first month of a Myanmar army crackdown on rebels in Rakhine state that began in August.
And more than 655,000 of the Muslim minority have fled across the border into Cox's Bazar district in southeastern Bangladesh since the army campaign began.
Why go after a little child if your actual target is an insurgent? - UN High Commissioner for Human Rights Zeid Ra'ad Al Hussein
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Myanmar has consistently denied committing atrocities in Rakhine, saying the crackdown was a proportionate response to the Rohingya militants who attacked police posts on August 25, killing around a dozen officials.
But Zeid said the evidence did not seem to support that, pointing to an upsurge in violence last year that had already prompted some 300,000 Rohingya to flee to Bangladesh.
His office, which has not been granted numerous requests to access northern Rakhine, published a report last February after speaking to refugees in Cox's Bazar who spoke of "horrific, horrific crimes, the hunting down of children and cutting their throats," Zeid said.
"My suspicion was that the first (smaller) operation was a dry run for the second," he said.
Suu Kyi responsible?
If the crackdown on the Rohingya was "organised and planned, it wasn't incidental violence that resulted from a counter insurgency operation," he said, also pointing out that civilians were clearly targeted.
"Why go after a little child if your actual target is an insurgent?" he asked.
The UN rights chief stressed that it would be up to a court to determine whether or not genocide had occurred, but "we don't believe you can exclude it and write it off as a possibility."
He voiced hope that perpetrators of the violence would eventually be held accountable.
"Those who are committing crimes now and believe they have security, they have the support of great powers, well times will change," Zeid said, warning that perpetrators "shouldn't feel so self-assured about this."
Asked about what level of responsibility for the violence should be attributed to Myanmar's civilian leader and Nobel Peace Prize laureate Aung San Suu Kyi, he said that too would be up to the courts to determine.
"To what extent could she have said the right thing or prevented this? You also have the crime of omission: if you're fully aware that these developments were taking place and did nothing to end them you could be held responsible," he said.
"A court will have to determine to what extent she had power over the military. She had power to recuse herself, to resign and not to be associated in any way with these developments, and the evidence will point eventually to who's culpable and who isn't."
