Philippine ex-president Rodrigo Duterte, facing International Criminal Court charges over his deadly narcotics crackdown, said "everything I did, I did for my country," his daughter told reporters Friday.

Sara Duterte, who is also her country's vice president, was briefing journalists at the detention center in The Hague after a long conversation with her father interned inside.

She also relayed that her father wants proceedings against him to move as quickly as possible, with the 80-year-old fearful of dying in custody.

"Everything I did, I did for my country. (I don't know) whether that statement is acceptable or not, but I want it out to the world," said Sara, relaying her father's words.

The vice president would not be drawn as to whether this amounted to an admission of responsibility for the crimes her father is facing at the ICC.

It was the first reported comments from Rodrigo Duterte since he appeared tired and dazed at an initial appearance at the ICC, which he attended by videolink and barely spoke.

In a video posted when Duterte arrived in the Netherlands last month to face the charges, he had said "I will be responsible for all of this."

Duterte stands accused of the crime against humanity of murder over his years-long campaign against drug users and dealers that rights groups said killed thousands.

In the prosecutor's application for his arrest, he said Duterte's alleged crimes were "part of a widespread and systematic attack directed against the civilian population in the Philippines".

"Potentially tens of thousands of killings were perpetrated," the prosecutor alleged of the campaign that targeted mostly poor men, often without proof they were linked to drugs.

In an interview with AFP earlier Friday, a lawyer for victims, Gilbert Andres, said loved-ones of those affected were seeking "truth and justice" from a potential trial.

The next hearing is scheduled for September 23, where the charges against him will be laid out, but Sara Duterte said her father was eager to get proceedings underway.

"He wants to go back to the Philippines. He said, 'I am an old man. I can die anytime. But I want to die in my country'," said Sara Duterte.