Philippine President Rodrigo Duterte has threatened government officials with the prospect of being thrown out of a helicopter mid-air if they are found to be corrupt, claiming he had done it before and had no qualms about doing it again.

The former prosecutor said he once hurled a Chinese man suspected of rape and murder out of a helicopter.

"If you are corrupt, I will fetch you using a helicopter to Manila and I will throw you out. I have done this before, why would I not do it again"? Duterte said during a speech to victims of a typhoon on Tuesday.

Duterte's latest threat comes just a few weeks after he admitted killing people during his 22 years as a mayor of Davao City, sometimes riding a motorcycle looking for "encounters to kill".

He said those killings were part of legitimate police operations, including a hostage incident. Some U.S. senators have warned Duterte he risks impeachment over his comments.

Duterte also said six people arrested last week during a seizure of more than half a tonne of methamphetamine, known locally as "shabu", in the capital were fortunate he was out of town.

"They were lucky I was not in Manila that time. If I had known there were that much shabu inside a house, I would definitely kill you", he said.

"Let's not make any drama, I will personally gun you down if nobody else will do it".

It was not immediately clear when or where the helicopter incident Duterte spoke of took place. His spokesman, Ernesto Abella, suggested it may not have actually happened.

"Let's just say, 'urban legend'," Abella told reporters including Reuters.

Zeid Ra'ad Al Hussein, the U.N.'s high commissioner for human rights has called for an investigation into Duterte's claims of killing people, to which Duterte last week responded by calling him "stupid", an "idiot" and a "son of a bitch" who should go back to school.

"The killings committed by Mr. Duterte, by his own admission ... clearly constitute murder," Hussein said in a statement. "It should be unthinkable for any functioning judicial system not to launch investigative and judicial proceedings when someone has openly admitted being a killer."