The UN human rights chief has called for a murder probe by Philippine authorities after president Rodrigo Duterte claimed to have killed people in the past.

High Commissioner for Human Rights Zeid Ra'ad Al Husseinand also urged authorities to look into the "shocking number of killings" under Mr Duterte's brutal crackdown on illegal drugs, which has left more than 6,000 people dead.

Mr Al Hussein said in a statement issued in Geneva that Philippine judicial authorities "must demonstrate their commitment to upholding the rule of law and their independence from the executive by launching a murder investigation".

He added it is "unthinkable for any functioning judicial system not to launch investigative and judicial proceedings when someone has openly admitted being a killer".

Such calls have enraged Mr Duterte and will likely face opposition from his officials.