Former Philippine president Rodrigo Duterte strongly defended his deadly drug war as he testified on Monday at a senate probe of the crackdown.

Police said the campaign left more than 6,000 people dead, but rights groups estimate tens of thousands of mostly poor men were killed by officers and vigilantes, often without proof they were linked to drugs.

The International Criminal Court (ICC) is investigating allegations that the killings were a state-sanctioned "crime against humanity".

"Do not question my policies because I offer no apologies, no excuses. I did what I had to do, and whether or not you believe it or not, I did it for my country," Duterte, 79, said in an opening statement at the public hearing.

"I hate drugs, make no mistake about it."

During the anti-drug campaign he launched shortly after taking office in 2016, Duterte ordered police to shoot dead suspects if officers believed their lives were in danger.

While the crackdown has been widely condemned and sparked an international investigation, only nine police officers have been convicted for killing drug suspects.

The drug war has continued under Duterte's successor, Ferdinand Marcos, though he has pushed for more emphasis on prevention and rehabilitation.

While the current and former president have had a bitter falling out, Marcos has stressed his government will not cooperate with the ICC investigation.

The Philippines quit the ICC in 2019 on Duterte's instructions, but the tribunal has said it has jurisdiction over killings before the pullout, as well as killings in the southern city of Davao when Duterte was mayor there, years before he became president.

Duterte claimed Monday that "drug-related crimes are on the rise again", with daily reports of "children being raped, people getting killed" because "the purveyors of this menace are back in business".

"The Filipinos are anxious and distressed," he added.