UN Secretary-General Ban Ki-moon has strongly condemned incoming Philippine president Rodrigo Duterte's apparent support of extrajudicial killings, voicing particular concern over his comments seen as justifying murdering journalists.

"I unequivocally condemn his apparent endorsement of extrajudicial killings, which is illegal and a breach of fundamental rights and freedoms," Ban said in a statement posted on the UN website.

"Such comments are of particular concern in light of on-going impunity for serious cases of violence against journalists in the Philippines," the statement quoted Ban as telling UN correspondents at a New York reception on Wednesday.

Duterte won last month's elections by a landslide largely due to an explosive law-and-order platform in which he pledged to end crime within six months by killing tens of thousands of suspected criminals.

He has since offered large bounties to security forces as well as the general public to kill drug traffickers.

"If they are there in your neighbourhood, feel free to call us, the police or do it yourself if you have (a) gun. You have my support," he told a weekend rally in his southern hometown of Davao city.

Duterte also told reporters last week that journalists who took bribes or engaged in other corrupt activities deserved to die.

"Just because you're a journalist you are not exempted from assassination, if you're a son of a bitch," he added.

Duterte raised the case of Jun Pala, a journalist and politician whose 2003 murder, like scores of other cases, has never been solved.

"I do not want to diminish his memory but he was a rotten son of a bitch. He deserved it," Duterte added.

Duterte's spokesman Salvador Panelo told AFP earlier this week the president-elect never said killing reporters was justified.

"What he said is there were instances where journalists were killed because of corruption.... He recognises the fact that there are many instances where journalists are killed because of their advocacies," Panelo added.

Duterte on Saturday also lambasted a local journalist, accusing him of "impertinence" when he questioned him about his health, asking to see his medical certificate.

"The son of a bitch . What if I asked him, 'How's the vagina of your wife? Is it smelly or not smelly?'," said Duterte.

The journalist, who was named by Duterte, later said the question had been asked by another reporter.

Also last week Duterte launched a profanity-laced tirade against the UN.

"Fuck you UN, you can't even solve the Middle East carnage... couldn't even lift a finger in Africa... shut up all of you."

He made the comments as he expressed irritation at criticism voiced by foreign and local media groups over his remarks about killing journalists.

Ban's comments were the most critical from any world leader about Duterte since he won the election. He is due to be sworn into office on June 30.