MANILA — President Rodrigo Duterte said Monday that his supposed election promise to ride a jet ski to challenge Chinese incursion in Philippine waters was a "pure joke" and that those believed it were "stupid."

Duterte in the lead-up to the 2016 polls said he would ride a jet ski to the Spratlys in the West Philippine Sea, plant the Philippine flag in a Chinese airport on reclaimed land there, and tell Beijing, "This is ours."

"Panahon sa kampanya ‘yan at saka iyong biro na ‘yon, we call it bravado," Duterte said of his statement five years ago.

"Iyong bravado ko was a pure campaign joke, at kung naniniwala kayo sa kabila, pati na siguro si [retired Supreme Court justce Antonio] Carpio, I would say you’re stupid,” he said in a taped speech.

(That was campaign period and that joke, we call it bravado. My bravado was a pure campaign joke, and if the other side believes that, maybe even Carpio did, I would say you're stupid.)

Appearing to jest, Duterte said he bought a secondhand jet ski, but spare parts for it have not arrived. He said there would be no gasoline station for the jet ski in the middle of the sea, and that waves would be as high as ships.

He said he does not know how to swim, and if his ride toppled into the ocean, "I would have been the late Rodrigo Duterte."

"Hindi ako mag-abot pa ng ilang oras, I would conk out it the middle of the sea... Takot akong mamatay,”

(I would not survive even a few hours, I would conk out it the middle of the sea. I am afraid to die.)

In 2016, he said he always wanted to be a hero and if China kills him, "Bahala na kayo umiyak dito sa Pilipinas."

(Cry for me here in the Philippines)

• Duterte says he 'never promised anything' on sea dispute vs China
• Duterte wants to be a hero in dispute vs China

Beijing refuses to obey an arbitral ruling that junked its historical claims to the South China Sea, within which is the West Philippine Sea.

Duterte shelved the 2016 ruling, as he sought investments and loans from China.

He has recently blamed the administration of his predecessor Benigno Aquino III for China's seizure of parts of the resource-rich waterway.

The President last week challenged Carpio to a debate that the former magistrate accepted and from which Duterte later backed out.

• Duterte tells Carpio: I'm not afraid of you, I just forgot you're not president