By Agence France-Presse
Jose Luis Gascon, chief of the Commission on Human Rights, also urged the police and justice ministry to cooperate with the ICC in its "preliminary examination" into whether Duterte has committed a crime against humanity in conducting his brutal crackdown on drugs.
ICC prosecutor Fatou Bensouda announced earlier this week that her office would analyse the alleged crimes carried out since July 1, 2016.
Since Duterte took office in mid-2016, he has overseen a crackdown that has left nearly 4,000 drug suspects dead at the hands of the police. But rights groups say the actual toll is three times that number.
"The CHR is ready, if requested, to assist in any way with the process of preliminary examination. The government, as a party to the Rome Statute, is duty-bound to fully cooperate with the ICC," Gascon said in a statement.
Gascon's comments came after Duterte on Friday declared himself beyond the jurisdiction of the probe, saying that while a previous administration had ratified the treaty establishing the ICC it had not become local law because of a technicality.
The outspoken leader has publicly called on police to kill drug suspects while vowing to protect officers from prosecution, maintaining the anti-crime operations are tough but legitimate.
Gascon's commission has also sought to investigate the drug war killings but has been hampered by a lack of cooperation from other government agencies like the police as well as limited resources.
Angered by the commission's criticism of Duterte's drug war, the president's congressional allies last year tried to slash its budget to just 1,000 pesos (about $20) until a public outcry forced them to reverse the move.
Duterte on Friday issued a veiled warning to ICC prosecutor Bensouda, telling her: "Do not go into that adventure. It's a messy one."
"You will know. You will get your comeuppance."
The ICC's initial inquiry is designed to help prosecutors determine if there is enough evidence of crimes that fall into its jurisdiction. It could lead to a full probe and eventually charges.