Duterte reportedly looked serious when he was telling the story and advised to forget about human rights.
US President Donald Trump arrived in the Philippines on Sunday for meetings with his Asia Pacific counterparts, the last stop of his Asian tour.
"Duterte last month declared the liberation of Marawi City from Islamist terrorists after 154 days of fighting, which killed more than 1,100 people, including 165 soldiers, and displaced almost 400,000 residents".
Local and global human rights groups claim thousands more have died as a result of the drug war's alleged extrajudicial killings, but the government has repeatedly denied this.
Estimates of how many people have been killed in Duterte's war on drugs range from 6,000 to 10,000.
Many Filipinos back Duterte, believing he is taking necessary measures to fight crime.
A senior USA official later tried to walk back Trump's comment, stating that Trump was merely acknowledging the U.S. is combating its own drug problem, not condoning violence and human rights violations.
"I'd be happy to slaughter them".
But the ICC has yet to respond and, despite some vocal critics in the West, Duterte goes into the Manila summits full of confidence that Trump and the others will effectively endorse his rule by not speaking against the killings.
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Before his arrival, Trump had met with President Rodrigo Duterte on the sidelines of the Asia Pacific Economic Cooperation meetings in Danang, Vietnam.
Those comments angered human rights organizations, who see Trump's meeting with Duterte as validation from the United States that the controversial President's crackdown on drug use in the Philippines is acceptable.
Duterte, 72, past year branded then-US president Barack Obama a "son of a whore" for criticising the drug war.
Duterte's past comments, though, could complicate how much Trump decides to embrace the world leader.
In Vietnam earlier on Sunday, Trump said he was prepared to mediate in the dispute over the South China Sea, where four Southeast Asian countries and Taiwan contest China's sweeping claims to the busy waterway.
'We will be extremely disappointed if Trump does not raise it, ' Amnesty's Philippine director, Jose Noel Olano, told reporters on Saturday.
It is also the host of the 31st Association of Southeast Asian Nations (ASEAN) Summit, which is set to welcome Trump and other world leaders.
Duterte can expect blanket support from his ASEAN colleagues, many of whom are also shadowed by human rights controversies.
Manila and Moscow signed a military deal on logistics, including a contract with a state-owned company for the supply of equipment, during the first-ever visit by a Russian defence minister to the Philippines last month.