Danang - Philippine President Rodrigo Duterte has said he stabbed a person to death as a teenager, in a defiant speech to promote his drug war ahead of a summit of world leaders in Manila.Speaking to the local Filipino community in the Vietnamese city of Danang on Thursday, Duterte also threatened to slap a UN rights rapporteur if he met her, and used obscene language to hit back at critics of his deadly drugs crackdown."When I was a teenager, I would go in and out of jail. I'd have rumbles here, rumbles there," said Duterte, who is in Danang for the Asia-Pacific Economic Cooperation (APEC) summit."At the age of 16, I already killed someone. A real person, a rumble, a stabbing. I was just 16 years old. It was just over a look. How much more now that I am president?"Duterte won last year's presidential elections after promising to eradicate illegal drugs with an unprecedented crackdown that would see up to 100 000 people killed.Since he took office 16 months ago, police say they have killed 3 967 people in the crackdown.Another 2 290 people were murdered in drug-related crimes, while thousands of other deaths remain unsolved, according to government data.Duterte, 72, remains popular with many Filipinos who believe he is making society safer.But critics at home and abroad warn that he is orchestrating a campaign of extrajudicial mass murder, carried out by corrupt police and hired vigilantes.He at times denies inciting police or others to kill, but also consistently generates headlines for his abusive language and incendiary comments defending the drug war.'Happy to slaughter'Duterte said last year he would be "happy to slaughter" three million drug addicts and branded then US president Barack Obama a "son of a whore" for criticising the drug war.Duterte also said in December last year that he had personally shot dead criminal suspects when he was mayor of southern Davao city to set an example for the police.His then spokesperson later sought to clarify the remarks, saying those killings were during a "legitimate police action". Killer, or killer jokesEsquire magazine quoted Duterte as saying in an interview before he became president that he "maybe" stabbed someone to death when he was 17 years old, in what may be a reference to the incident described in Danang.In an election campaign rally Duterte also said he was expelled from college for shooting a fellow student who was insulting him. The victim reportedly lived.Duterte's aides have repeatedly told journalists not to believe everything the president says, cautioning that he often jokes or indulges in "hyperbole".His new spokesperson, Harry Roque, indicated that may be the case with his stabbing-to-death claim."I think it was in jest. The Pres uses colourful language when w Pinoys (Filipinos) overseas, [sic]" Roque said in a text message.In Danang, Duterte also targeted the United Nations' special rapporteur on extrajudicial killings, Agnes Callamard, who has been a frequent critic of the drug war."This rapporteur," he said, after referring to Callamard by name. "I will slap her in front of you. Why? Because you are insulting me."Mass killings campaignDuterte's latest comments come ahead of him hosting US President Donald Trump and other leaders for the Association of Southeast Asian Nations (ASEAN) summit.Trump is due to fly into Manila from Vietnam along with many other leaders on Sunday evening, ahead of two days of talks.Trump has praised Duterte's handling of the drug war, telling the Philippine leader in a telephone call in April that he was doing a "great job".-Read: Trump praises Duterte's 'great job' in Philippine drug war: reportsHuman rights campaigners have said the summit will be a public relations coup for Duterte, with Trump and other leaders expected to ignore the drug war controversy."Duterte will enjoy the gift of tacit silence from East Asian leaders on his murderous drug war during the upcoming summit," Human Rights Watch deputy Asia director Phelim Kine told AFP."The summit messaging will be of feel-good mutual economic win-wins that will ignore the fact that Duterte has inflected a campaign of mass extrajudicial killings.