The US government has expressed concern over extrajudicial killings of suspected drug dealers and users in a bloody crackdown overseen by Philippine President Rodrigo Duterte and urged his government to ensure law enforcement efforts comply with human rights obligations.

US State Department spokesman Elizabeth Trudeau on Monday also criticised recent remarks by Duterte about the US ambassador in Manila and said Philippine officials have been asked to clarify them.

The president recently referred to US Ambassador Philip Goldberg using a profanity and accused him of meddling in the Philippine elections.