The arrest of former Philippine president Rodrigo Duterte is an important moment for alleged victims of his "war on drugs", International Criminal Court Chief Prosecutor Karim Khan said on Wednesday.

The fact the ICC arrest warrant has been carried out "is important to victims. It means a lot, I think, to victims," said Khan in a statement.

"Many say that international law is not as strong as we want, and I agree with that. But as I also repeatedly emphasise, international law is not as weak as some may think," added Khan.

"When we come together... the rule of law can prevail," said the prosecutor.