The Center of Mosque Studies - Zeid Ra’ad Al-Hussein told the US National Public Radio that his team on the ground was "receiving accounts of indiscriminate shootings, summary executions, arbitrary arrest, enforced disappearances, rape and other forms of sexual violence and torture.”
He added:"This seems to be continuing as we speak today.”
He also repeated his previous assertion that the situation in Myanmar "remains a textbook example of ethnic cleansing.”
UN doctors have warned of seeing dozens of women with injuries consistent with sexual violence in recent weeks.
More than 350 people had been referred for "life-saving care” relating to gender-based violence - a broad term that includes rape, attempted rape and molestation- since the start of the crisis.
Refugees are continuing to flee to Bangladesh which has seen an influx of more than 580,000 refugees since August 25, when Myanmar security forces began a scorched-earth campaign against Rohingya villages.
The Myanmar government claims its military was acting in retaliation to attacks by Muslim insurgents.
But the response has been almost universally condemned by the international community.