(2nd UPDATE) US President Barack Obama on Monday called a planned meeting with Rodrigo Duterte into question after the Philippine leader launched a foul-mouthed tirade against him.

"He's a colorful guy," Obama joked on the eve of the scheduled face-to-face, before pointedly noting: "I always want to make sure if I'm having a meeting that it's actually productive."

"What I've instructed my team to do is to talk to their Philippine counterparts to find out [if] is this in fact a time where we can have some constructive productive conversations," he said.

"Obviously the Filipino people are some of our closest friends and allies, and the Philippines is a treaty ally of ours. But I always wanna make sure that if I have a meeting that it's actually productive and we are getting something done.

"We recognize the significant burden the drug trade plays not just in the Philippines but around the world. Fighting narco trafficking is tough. But we will always assert the need to have due process and engage in that fight against drugs in a way that is consistent with international norms," he added.

"And so, undoubtedly, if and when we have a meeting, that is something brought up. And my expectation, my hope is that it could be dealt with constructively. But I'll have my team discuss this, I've got a whole bunch of folks that I'm going to be meeting with in the course of the next several days," Obama said.

"So I expect that will continue, but I wanna make sure the setting is right, the timing is right for us to have the best conversation possible."

Duterte called Barack Obama a "son of a whore" on Monday as he vowed not to be lectured by the US leader on human rights when they meet in Laos.

The acid-tongued Duterte bristled at warnings he would face questioning by the US president over a crime war in the Philippines that has claimed more than 2,400 lives in just over two months.

"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 a news conference shortly before flying to Laos to attend the summit.

"We will be wallowing in the mud like pigs if you do that to me."

READ: Duterte hits US anew before leaving for ASEAN Summit

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Duterte was due to hold a bilateral meeting with Obama on Tuesday afternoon on the sidelines of a summit of global leaders hosted by the Association of Southeast Asian Nations (ASEAN) in Vientiane, the Lao capital.

Duterte was elected in May after a promise to wage an unprecedented war on illegal drugs that would see tens of thousands of suspects killed.

Official figures released Sunday show that, since Duterte took office on June 30, over 2,400 people have been killed in police anti-drug operations and by suspected vigilantes.

Duterte has angrily rejected criticism from the Catholic Church, human rights groups, legislators and the United Nations.

Duterte vowed Monday the bloodbath would continue as he pursued his goal of eradicating illegal drugs in the Philippines.

"More people will be killed, plenty will be killed until the last pusher is out of the streets. Until the (last) drug manufacturer is killed, we will continue and I will continue," he said.

Duterte insisted he would not take orders from the United States, a former colonial ruler of the Philippines, and did not care about how he was perceived.

"I don't give a s*** about anybody observing my behavior," he said.

Duterte also used vulgar language to accuse his domestic critics of wanting to please the United States.

"There are others who have the mental capacity of dogs who lap at the ass of the Americans," he said in reference to his critics.

Duterte is notorious for using offensive language.

During the election campaign Duterte described the US ambassador to Manila as a "son of a whore" and being homosexual.

This was in response to the ambassador's criticism of Duterte for making a joke about wanting to rape a "beautiful" Australian missionary who was killed in a Filipino jail. - with a report from Reuters