Philippine President Rodrigo Duterte risks impeachment after he admitted "personally" killing criminals when he was the mayor of Davao City, said two Philippine senators.

The firebrand leader boasted to business leaders on Monday that as Davao City mayor he used to prowl the streets on a big motorcycle looking for "an encounter to kill" just to show to local law enforcers he can do it.

"I used to do it personally," Duterte said. "If I can do it, why can't you?"