Zeid Ra'ad Al Hussein said there were allegations of "killing by random firing of bullets, use of grenades, shooting at close range, stabbings, beatings to death
and the burning of houses with families inside".
UN Secretary-General Antonio Guterres had earlier said "ethnic cleansing" was taking place in Myanmar.
The UN has said over 6,80,000 Rohingya refugees have crossed over to Bangladesh since August 25th allegedly due to atrocities committed by the Myanmar Army.
Last week, Pope Francis had visited Myanmar and Bangladesh to find a solution to the crisis which has spiralled out of control in recent months.