VIETIANNE (AFP/REUTERS) - US President Barack Obama urged Philippine leader Rodrigo Duterte on Thursday (Sept 8) to conduct his crime war "the right way", after 3,000 people were killed in the crackdown in just over two months.

"As despicable as these (crime) networks may be and as much damage as they do, it is important from our perspective to make sure that we do it the right way," Mr Obama told reporters when asked about his conversation with Mr Duterte on the sidelines of a regional summit in Laos.

"Because the consequences of when you do it the wrong way are innocent people get hurt and you have a bunch of unintended consequences that don't solve the problem."

Relations between long-time allies, the US and the Philippines, saw a spectacular setback this week after firebrand politician Duterte branded Mr Obama a "son of a whore".

The outburst on Monday was in response to being told that Mr Obama planned to raise concerns about his war of drugs.

"You must be respectful. Do not just throw away questions and statements. Son of a whore, I will curse you in that forum," Mr Duterte told reporters shortly before flying to Laos. "We will be wallowing in the mud like pigs if you do that to me."

At the press conference marking the end of his trip to Laos, Mr Obama said he was unfazed by Mr Duterte's jibe.

"I don't take these comments personally because it seems as if this is a phrase he's used repeatedly including directed at the pope and others," he said, adding that such choice words were "a habit, a way of speaking for him".

Mr Duterte has branded Pope Francis, the US ambassador to Manila and the United Nations as "sons of whores".

However Mr Obama cancelled a meeting with Mr Duterte scheduled for Tuesday because of the outburst.

They met briefly on Wednesday night before a leaders' dinner, but only exchanged "pleasantries", according to the White House.

Mr Duterte was elected to office in a landslide victory this year after pledging to kill 100,000 people in an unprecedented war on crime. He vowed during the campaign that so many bodies would be dumped in Manila Bay that the fish there would grow fat from feeding on them.

He has also repeatedly promised to protect police from prosecution if they are charged over the deaths and insisted human rights cannot get in the way of his war. He has said the Philippines is in danger of becoming a "narco state", and eliminating drugs in society is the top priority of his administrations.

On the day he was sworn into office, on June 30, Mr Duterte urged people living in a Manila slum to kill drug addicts in their community.

His handpicked police chief Ronald dela Rosa last month called for drug addicts to kill traffickers and burn down their homes.

The United Nations special rapporteur on summary executions has warned incitement to kill is a crime under international law.

Despite growing condemnation of the comments and what rights groups say are rampant extrajudicial killings, Mr Duterte has vowed to continue.

"More people will be killed, plenty will be killed until the last pusher is out of the streets," he said on Monday.

With months left in office, Mr Obama also said he was not ready to give up on an eight-year-old promise to close the prison at Guantanamo Bay.

"I am not ready to concede that it may still remain open, because we're still working diligently to continue to shrink the population," he said.

He urgently wants to close the facility before he leaves office at the start of next year but has been continually thwarted by Republican lawmakers.

Still, the United States has in recent months accelerated the rate at which detainees who have been approved for transfer are released from the facility.

Mr Obama said the controversial prison served as a "recruitment tool" for terrorist organisations and was a waste of money.

"As we continue to shrink the population to the point where we're looking at 40 or 50 people and are maintaining a multi-million dollar operation to house these people, the American people are asking why should we spend this money on this when it could be spent on other things," he told reporters.