Former Philippine President Rodrigo Duterte says he killed four suspects in clamping down on illegal drugs.

During his term in office, Duterte led a hard-line crackdown on narcotics that became known as the "war on drugs." The government has said more than 6,000 people were killed in the campaign. Relatives of the dead claim that the state carried out extrajudicial murders.

Congress is investigating the killings. Duterte attended a hearing at the parliament for a second time on Wednesday.

He said that before becoming president, he intentionally killed four suspects after entering a hideout of a drug crime group.

He said he wants the International Criminal Court to "start the investigation tomorrow," adding that the issue has been pending for many years.

The country withdrew from the ICC while Duterte was in office.

In response to Duterte's remarks, Philippine Executive Secretary Lucas Bersamin issued a statement. It says that if the ICC refers the matter to Interpol, and places Duterte on a wanted list, the Philippine authorities will be obliged to cooperate.

This is the first time that the Philippine government has shown its readiness to cooperate in an investigation by the ICC into the matter.