ICC judges ruled that Duterte posed a flight risk and could influence witnesses if freed, arguing that this outweighed any medical problems suffered by the 80-year-old.

"The Chamber finds that Mr Duterte's detention continues to remain necessary," the ICC said in a heavily redacted ruling dated October 10.

ICC prosecutors have charged Duterte with three counts of crimes against humanity, alleging his involvement in at least 76 murders as part of his "war on drugs".

The first count concerns his alleged involvement as a co-perpetrator in 19 murders carried out between 2013 and 2016 while Duterte was mayor of Davao City. The second count relates to 14 murders of so-called "High Value Targets" in 2016 and 2017 when Duterte was president. And the third charge is about 43 murders committed during "clearance" operations of lower-level alleged drug users or pushers.

These took place across the Philippines between 2016 and 2018, the prosecution alleged.

Duterte was arrested in Manila on March 11 and flown to the Netherlands that same night, and has been held at the ICC's detention unit at Scheveningen Prison since. He followed his initial hearing by video link, appearing dazed and frail and barely speaking.

His lawyer Nicholas Kaufman has said Duterte is not able to stand trial "as a result of cognitive impairment in multiple domains".

Kaufman has urged the ICC to postpone proceedings against Duterte indefinitely.

Reacting to Friday's ICC decision, Kaufman told AFP: "The Defence considers that the decision to dismiss unprecedented State guarantees for a debilitated and cognitively impaired 80-year-old, kept from the public eye for more than six months, to be erroneous."

"An appeal was filed a week ago," he added.

'Flight risk'

The ICC found Duterte "continues to pose a flight risk and detention is necessary to ensure his presence during the pre-trial proceedings and possible trial".

Judges pointed to Duterte's "necessary political contacts" and "a network of support", including his daughter Sara Duterte, currently vice-president of the Philippines. This network "may help him abscond", judges said.

The ICC also found there was a risk that Duterte and his network "would pose a threat to [potential] witnesses, either directly or indirectly through his supporters."

This also argued for his continued detention, the court ruled.

Finally, the judges said that medical reports provided by the defence, which they say show Duterte suffers from cognitive decline, were insufficient to allow his release.

"The Chamber considers that the [...] documents do not indicate how Mr Duterte's alleged physical condition or cognitive impairment negate the risks identified above," the ICC said.

However, judges stressed that this assessment related only to the issue of interim release, not to the determination of whether he was fit to stand trial.

The ruling "is without prejudice to any further determination and decision on the separate matter of Mr Duterte's fitness to participate in the present proceedings and the confirmation of charges hearing," the document said.

The "confirmation of charges" hearing is the next step in the case, where lawyers will argue whether there are sufficient grounds for an actual trial.

This has been postponed indefinitely while judges decide whether Duterte's alleged health problems have rendered him unfit to take the dock.

Paolo Duterte, his son, described the decision as "a gross and disgraceful miscarriage of justice".

"It is not law, it is a political theater," said the younger Duterte, a member of the House of Representatives.