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.
The report found that most victims are young poor urban males and that police, who do not need search or arrest warrants to conduct house raids, systematically force suspects to make self-incriminating statements or risk facing lethal force.