MANILA - President Rodrigo Duterte issued a stark challenge to United States, European Union and the United Nations on Thursday that they would end up humiliated and outsmarted if they accepted an invitation to investigate his war on drugs.

Speaking to an audience of business people at a conference in the capital Manila, Duterte said he was open to an outside probe into alleged extra-judicial killings, but on the condition that after he was questioned, he had the right to be heard.

"I'll play with you. I'm very sure they cannot be brighter than me. I will ask five questions that will humiliate you," Duterte said. "Watch out for that, it will be a spectacle."

In Duterte's speech, which was televised, he said it was necessary to cleanse the streets of drug pushers and rescue the next generation of Filipinos from the scourge of narcotics.

Duterte, 71, won the hearts of millions of Filipinos with his outrageous, at times comical speeches and man-of-the-people style in the run-up to a May election.

• READ: Duterte's shooting tip: Hit head, heart or balls

He won by a huge margin after campaigning almost entirely on promises to wipe out drugs and crime.

Nearly 2,300 people have died in the war on drugs since the campaign started on June 30, according to police, of which 1,566 were drug suspects killed in police operations.

Police had previously said there had been more then 3,600 deaths, but have since concluded that many of that number were homicides and murders unrelated to illegal narcotics.

Opinion polls for Duterte's first 90 days in office suggest he remains popular, with a Pulse Asia survey on Wednesday showing he had the trust of 86 percent of 1,200 Filipinos surveyed.

Duterte said on Wednesday he had officially invited a United Nations special rapporteur on extrajudicial executions to investigate the drug killings.

• READ: U.N. rapporteur welcomes invitation to probe drug killings