Its report levelled heavy criticism at President Rodrigo Duterte's government.
His drugs crackdown has been marked by high-level rhetoric that can be seen as "permission to kill", the report said, urging an independent investigation.
The administration has in the past rejected all criticism of its policies and denies the killings are illegal.
Official figures show more than 8,000 people were killed in the war on drugs since Mr Duterte took office in 2016. Other estimates put the figure three times as high.
