Philippine police have said they have killed roughly 4,100 suspects who fought back during arrest, but rights groups allege the actual number is three times higher and accuse the authorities of murder

Philippine police said Thursday they had shot dead 13 drug suspects, just days after President Rodrigo Duterte moved to take the country out of the International Criminal Court over its inquiry into his deadly drug war.

The suspects were killed Wednesday in the northern province of Bulacan, an official statement said, an area where police have previously launched lethal crackdowns on illegal drugs.

"Bulacan police are continuously and relentlessly implementing their intensified campaign against illegal drugs," the statement said, adding there had been more than 100 arrests.

The war crimes tribunal, based in The Hague, last month launched a preliminary inquiry into Duterte's bloody crackdown on narcotics, amid allegations Philippine security forces may have committed crimes against humanity.

Philippine police have said they have killed roughly 4,100 suspects who fought back during arrest, but rights groups allege the actual number is three times higher and accuse the authorities of murder.

The tribunal opened in 2002 to try abuses in countries where national courts cannot or will not prosecute. Manila in 2011 ratified the Rome Statute that created the court.

Manila gave official notice to the United Nations last week that it would withdraw, days after Duterte announced his country would quit the court over alleged "baseless, unprecedented and outrageous attacks" against his government's rights record.

Foreign Secretary Alan Peter Cayetano said last Friday the Philippines was pushing back against "the well-orchestrated campaign to mislead the international community, to crucify President Duterte... by distorting the human rights situation in the country".

The tribunal has urged Manila to reconsider its decision, adding that officially quitting the court requires a year's notice and does not preclude its preliminary inquiry into the drug war killings, which have drawn international concern.

Duterte, who is buoyed by high popularity ratings at home, has fiercely defended the drug war as a battle to bring safety to the nation's 100 million people.

He has frequently urged authorities to kill drug suspects while promising to protect police from legal sanction.