US President Barack Obama yesterday urged Philippine leader Rodrigo Duterte to conduct his crime war "the right way", as the pair clashed at an Asian summit over the crackdown that has claimed 3,000 lives.

The showdown was a fitting climax to a brutal few days of diplomacy that began with the famously acid-tongued Duterte branding Obamaa "sonofawhore".

After warnings from Duterte that he would not be lectured on his crime war - which is seeing police and shadowy assassins kill an average of 44 people a day - Obama urged the Philippine leader to respect the rule of law.

Duterte has said the Philippines is in danger of becoming a "narco state", and eliminating drugs in society is the top priority of his administration.

Obama's call came shortly after Duterte gave what diplomats said was a "fiery" address to leaders of the 18-nation East Asia group, including Obama.

Veering off his prepared speech, Duterte launched into a tirade about US military killings in the Philippines whenit was an American colony from 1898 to 1946, according to three diplomats AFP spoke with who were in the room.

"The Philippine president showed a picture of the killings of American soldiers in the past and the president said: 'This is my ancestor they killed. Why now we are talking about human rights," an Indonesian delegate said.

Duterte set the tone for the week when, just before flying to Laos on Monday, he launched a barrage of insults at Obama in response to the US president's plans to question him over his war on drugs.

"You must be respectful. Do not just throw away questions and statements. Son of a

whore, I will curse you in that forum," Duterte told reporters shortly before flying to Laos.

At the press conference marking the end of his trip to Laos, Obama said he was unfazed by Duterte's slur. He added that such choice words were "a habit, a way of speaking for him".

Duterte has branded Pope Francis, the US ambassador to Manila and the United Nations as "sonsofwhores".