0931 GMT September 23, 2018
The move comes two days after Duterte announced his nation would quit the court, and triggered warnings Friday from a top tribunal official that it would harm global efforts to end impunity for the world's worst crimes, AFP reported.
The ICC, based in The Hague, last month launched a preliminary inquiry into Duterte's bloody crackdown on narcotics, amid allegations that actions by Philippine security forces amount to crimes against humanity.
Philippine police say they have killed roughly 4,000 suspects who fought back during arrest, but rights groups allege the actual number is three times higher and accuse the authorities of murder.
The ICC opened in 2002 to try abuses in countries where national courts cannot or will not prosecute. Manila joined the Rome Statute in 2011.
But on Thursday the Philippines said in a letter to the UN, which oversaw negotiations to found the court that it was pulling out of the Rome Statute the tribunal's founding guidelines.
"The decision to withdraw is the Philippines' principled stand against those who would politicize and weaponized human rights," the letter said.
Foreign Secretary Alan Peter Cayetano, speaking from Manila, said the Philippines was quitting due to "the well-orchestrated campaign to mislead the international community, to crucify President Duterte... by distorting the human rights situation in the country".
But the tribunal urged Manila to reconsider its decision, and the president of its governing body, the Assembly of State Parties (ASP), said he deeply regretted the move.
"A state party withdrawing from the Rome Statute would negatively impact our collective efforts towards fighting impunity,” said ASP president, O-Gon Kwon.
"The ICC needs the strong support of the international community to ensure its effectiveness. I encourage the Philippines to remain as a party to the Rome Statute."
Officially quitting the court requires a year's notice and does not stop the ICC from continuing its investigation of the killings, which have drawn international concern.
Should the Philippines fully withdraw from the court it would not be the first to do so, as Burundi became the first ever nation to leave in October 2017.
The Philippines said in its letter that it "affirms its commitment to fight against impunity for atrocity crimes", despite its withdrawal.
Duterte, who is buoyed by high popularity ratings at home, has fiercely defended the drug war as a battle to bring safety to the nation's 100 million people.
He has frequently urged authorities to kill drug suspects while promising to protect police from legal sanction.