MANILA (REUTERS) - Families of people killed in the Philippines' war on drugs are hopeful that a formal investigation into possible crimes against humanity approved by judges from the International Criminal Court (ICC) earlier this week will result in justice for the victims.

Rights groups say Philippine President Rodrigo Duterte incited deadly violence, and accuse the police of murdering unarmed suspects on a massive scale as part of the anti-narcotics campaign launched in 2016.

"I am grateful that our situation and the killing of my brother are being paid attention to," Ms Mary Jane Gerango, 40, told Reuters in Manila. "Our hope is that our family gets justice and those who are at fault must be held accountable."

Ms Gerango's younger brother was killed by police in September 2016 after being tagged as an alleged drug dealer.

The Philippine National Police issued a statement saying the accusations were repeatedly proven to be false, and that the force "neither condones nor covers up abuses and other forms of wrongdoings in our ranks".

"Our aggressive campaign against illegal drugs will continue," the office of the police spokesman said noting a 64 per cent fall in the drug-related crimes in the past five years.

Remains of seven Filipinos killed under Mr Duterte's bloody war on drugs were exhumed on Friday (Sept 17) for cremation, after leases in public cemeteries north of the capital expired.

The authorities have killed more than 6,100 suspected drug dealers and users since Mr Duterte took office in June 2016. Rights group say the police summarily executed suspects, which the policy deny, saying they acted out of self-defence in sting operations.

Judges in The Hague said on Wednesday that prosecutors' materials showed the anti-drug campaign "cannot be seen as a legitimate law enforcement operation", but rather a systematic attack on civilians.

The Philippine government on Thursday said it will not cooperate with the ICC's probe nor allow any investigators into the country.

"The opening of this investigation is a way, not only to get justice but also for them to heal and get closure," Catholic priest Flavie Chalaf said after blessing the remains of the exhumed victims.

"If it is true that you are clean, why must you hide?" he said, addressing Mr Duterte's government.