Rodrigo Dutertes lead lawyer said Sunday there was a "compelling" argument to throw out the International Criminal Court case against the former Philippines president before it even comes to trial.

Nicholas Kaufman told AFP in an interview in The Hague he hoped to stop the case before the ICC confirms the charges against Duterte by arguing the court cannot exercise its jurisdiction.

He said the Philippines withdrawal from the court had become effective well before an investigation was authorised.

Duterte, 80, faces a charge of crimes against humanity for murder over his "war on drugs" that claimed the lives of thousands of mostly poor men, often without proof they were linked to drugs.

British-Israeli lawyer Kaufman, 56, said: "Coming back to the jurisdictional point, obviously you dont need to be the dean of a law faculty to realise that thats going to be a huge issue at pre-trial."

"I think that the jurisdictional argument is compelling as defence counsel. I believe that it should succeed and I would be hugely disappointed if it doesnt succeed," he added.

"We hope to persuade the judges pre-trial that it (the court) cannot exercise its jurisdiction over the case. There wont be a confirmation-of-charges hearing if the judges rule in our favour."

A confirmation of charges hearing, where prosecutor and defence first lay out their evidence, is currently scheduled for September 23.

The issue of jurisdiction is key in this case as the Philippines withdrew from the ICC in 2019.

However, when the court issued its arrest warrant for Duterte, it noted that the alleged crimes took place while the country was still an ICC member.

"As the alleged conduct has taken place between 1 November 2011 and 16 March 2019 on the territory of the Philippines, it falls within the Courts jurisdiction," the ICC said.

The ICC chief prosecutors application for his arrest said Dutertes alleged crimes were "part of a widespread and systematic attack directed against the civilian population" in the Philippines.

The families of victims of his war on drugs see the ICC case as a long-awaited chance for justice.

Another likely critical issue for the defence will be Dutertes arrest on March 11 and his rapid handover to the ICC in The Hague.

"I view it as a kidnapping, nothing more or less. Its an extrajudicial rendition. He was given no due process, just slung over to the Hague," Kaufman told AFP.

"This was in complete contravention of Philippines law."

The former presidents sudden detention came amid a spectacular deterioration in relations between the two most powerful families in Philippines politics.

The Duterte and Marcos clans teamed up to win an election landslide in 2022 that saw Ferdinand Marcos become president and Sara Duterte -- daughter of the imprisoned Rodrigo -- vice-president.

Sara Duterte has since been impeached on charges that include an alleged assassination plot against the president.

Rodrigo Duterte "should have been brought before a judge before he was thrown onto a plane and dumped in The Hague. That didnt happen. As Ive said already, this is a political hit job," said Kaufman.

"The politics in that country basically ended up in a situation where they needed to get him out of the picture. The incumbent government did not want him in the picture anymore."

Not easy for anyone

Kaufman said he was visiting his client virtually every day in the ICC detention centre, a couple of kilometres from the beach in the suburb of Scheveningen.

Duterte was "adjusting to the reality of prison life. Thats not easy for anyone," according to his lawyer.

However, the former president was in "good spirits," said Kaufman, whose previous clients at ICC include former Congolese rebel leader Jean-Pierre Bemba and Aisha Kadhafi, daughter of the deceased Libyan dictator.

Kaufman said he was concerned that the ICC, currently under fire from all sides and even US sanctions, would be reluctant to give up such a high-profile case.

"My only fear is that this court is starved of cases at the present moment and might be loath to let a case like that go, to slip through its hands."