Duterte said he would swear at Obama at a scheduled meeting if the president raises the issue extrajudicial killings of drug suspects in the Philippines.
Around 2,400 suspected drug dealers and users have been killed without trial, in a drug war Duterte launched after taking office in June. His efforts have won public support, but the killings have concerned Washington and United Nations officials.
Obama is scheduled to meet with Duterte in Laos at the summit of the Association of Southeast Asian Nations, but hinted Monday he might cancel the meeting. Obama said the lack of due process in Duterte's anti-drug campaign is "something that's going to be brought up" if the two meet.