Philippine President Rodrigo Duterte has called Barack Obama a "son of a whore", warning the US leader not to raise human rights concerns with him.
Mr Duterte was asked by a reporter how he intends to respond should Mr Obama ask about his war on the drug trade.
Hundreds of people have died in anti-drug operations since Mr Duterte won election despite international alarm.
He and Mr Obama are due to meet on Tuesday in Laos on the sidelines of an Asean conference.
Mr Duterte, who took office in June, has sanctioned the killing of drug dealers to try to wipe out the illegal drugs trade in the Philippines.
The UN has repeatedly condemned his policies as a violation of human rights. The Roman Catholic church, the dominant religion in the Philippines, has also criticised Mr Duterte.
But Mr Duterte said he is not concerned about the opinions of those observing his actions, adding that he would not take orders from the US, a former colonial ruler of the Philippines.
"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 a news conference, referring to President Obama.
"We will be wallowing in the mud like pigs if you do that to me," Mr Duterte said.
Some 2,400 people have been killed in his war on drugs since he took office and he said there were more deaths to come.
"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.
In August, two UN human rights experts said Mr Duterte's directive for police and the public to kill suspected drug traffickers amounted to "incitement to violence and killing, a crime under international law".
Asked whether he would still meet Mr Duterte and raise the issue of extrajudicial killings, Mr Obama said he was assessing whether a meeting would still be constructive.
"I always want to make sure that if I'm having a meeting that it's actually productive, and that we're getting something done,'' he said.