The International Criminal Court on Monday postponed "until further notice" a crimes against humanity hearing concerning former Philippines president Rodrigo Duterte, while judges weigh his fitness to take part.

Duterte was scheduled to appear at the ICC on 23 September to hear the charges over his years-long campaign against drug users and dealers that rights groups say killed thousands.

But the court decided to adjourn the hearing until it could rule on a defense motion that the 80-year-old was "not fit to stand trial".

"Litigation regarding Mr Duterte's fitness to participate in the pre-trial proceedings is ongoing in connection with the Defense's Adjournment Request," said the court in a statement.

Hence the court decided that a "limited postponement of the hearing on the confirmation of charges is warranted to allow sufficient time to adjudicate" on the defense request.

The three-judge panel was split on the decision, with one dissenting.

The postponement will be "limited to the time strictly necessary to determine whether Mr Duterte is fit to follow and participate in the pre-trial proceedings", the ICC said.

Duterte was arrested in Manila on 11 March, flown to the Netherlands that same night and has been held at the ICC's detention unit at Scheveningen Prison since.

At his initial hearing, he followed by videolink, appearing dazed and frail, barely speaking.

He is the first Asian former head of state charged by the ICC.

A lawyer representing relatives of those killed in Duterte's drug war said they were disappointed with the ICC decision.

"We expected that Duterte would do this, but the fact that he was able to convince the pre-trial chamber to postpone that indefinitely is scary," Kristina Conti told AFP.

"Some of us were very angry because there was a schedule and everyone was ready."

Conti urged ICC judges to "see through the lies" and proceed to trial, adding: "we have no other hope than to hope" that he will face justice.

Duterte stands accused of the crime against humanity of murder over his years-long campaign that prosecutors allege were "part of a widespread and systematic attack directed against the civilian population in the Philippines".