Bangkok: The United Nations has called on Philippine authorities to investigate controversial president Rodrigo Duterte for murder after he bragged about killing people last week.

Zeid Raad al-Hussein, the UN's High Commissioner for Human Rights, also urged investigations into the "appalling epidemic of extra-judicial killings" under Mr Duterte's deadly crackdown on drugs that has left almost 6000 people dead.

In unprecedented criticism of a serving national leader by the world body, Prince Zeid said children as young as five have become victims of the violence, adding he was surprised there has been no sign of prosecution of those responsible.

"The perpetrators must be brought to justice, sending a strong message that violence, killings and human rights violations will not be tolerated by the state and that no one is above the law," Prince Zeid said.