MANILA, Philippines — The Philippine Senate said Monday it will conduct a formal probe of ex-president Rodrigo Duterte's arrest and swift handover last week to the International Criminal Court, which is to try him for alleged crimes against humanity.

The 79-year-old, the first Asian former head of state charged by the ICC, stands accused of the crime against humanity of murder over his years-long campaign against drug users and dealers that rights groups have said killed thousands.

The probe was initiated by Senator Imee Marcos, sister of President Ferdinand Marcos but a close friend of Duterte's eldest daughter, Vice President Sara Duterte.

The two families have had a spectacular falling out since Marcos teamed with Duterte to win an election landslide in 2022. The latter has since been impeached on charges that include an alleged assassination plot against the president.

"As chairperson of the Senate Committee on Foreign Relations, I am calling for an urgent investigation into the arrest of former President Rodrigo Roa Duterte, an issue that has deeply divided the nation," Imee Marcos said in a statement Monday.

"It is imperative to establish whether due process was followed and to ensure that his legal rights were not just upheld but protected," she said, adding: "Our sovereignty and legal processes must remain paramount."

Duterte was arrested at Manila airport on 11 March after a brief trip to Hong Kong and flown to the Netherlands just hours later where he was turned over to the ICC.

The Senate has set a public hearing for Thursday and invited top police and other government officials to give evidence.

Imee Marcos has tracked a course largely independent from her brother on many issues, though she is running for re-election under the administration's ticket in the 12 May midterm elections.

Hours after Duterte's arrest, Imee Marcos warned at a news conference that it could "only lead to trouble".

Separately, a veteran international lawyer with ICC experience has been tapped to join the former president's defense team.

Nicholas Kaufman, a British-Israeli national, has previously represented clients at The Hague including former Congolese rebel leader Jean-Pierre Bemba and Aisha Kadhafi, daughter of the deceased Libyan dictator.

"The president (has) already appointed Nicholas Kaufman as his lawyer," Vice President Duterte confirmed at a press briefing outside the Hague, according to a transcript made public Sunday by her office.

"We had a meeting with him yesterday, and then we will have a meeting in person when he arrives this weekend," she told reporters after her father's Friday appearance.

In an email to AFP, Kaufman said he was "honoured to have been asked to assist former President Duterte in composing his defence team in which my future role is yet to be precisely determined".

"Indeed, I look forward to denouncing the State-sponsored abduction of the former President to a case in The Hague devoid of jurisdiction".