Pillay's office said it had evidence to suggest civilians in South Darfur state in Sudan were the target of recent rebel attacks.
"There has been a disproportionate use of force by armed groups in areas in South Darfur that are not military targets," she said in a statement from Switzerland. "There must be an immediate halt to attacks on unarmed civilians."
Her office said it couldn't provide an estimate of the number of people killed, though at least 50,000 people have been displaced by the conflict.
"I urge the authorities to protect civilians and hold to account those who have committed grave breaches of human rights and humanitarian laws," she said.
A joint mission of the African Union and United Nations, known by the initials UNAMID, said last week it was prohibited by regional authorities from sending peacekeepers into the area to survey the damage.
UNAMID reported the violence erupted last week in Um Gunya, a village in the Darfur region.