MANILA — A possible investigation of the International Criminal Court (ICC) into President Rodrigo Duterte's deadly war on drugs is "no big deal," Malacañang said Thursday, even as families of the victims cheered the move.

In what human rights groups described as a landmark step towards justice, the ICC prosecutor asked the court on Monday to allow a full investigation into the killings in the anti-narcotics campaign that Duterte unleashed when he took office in 2016.

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Palace spokesman Harry Roque said the ICC prosecutor's move was mostly based on media reports of the killings.

"After we saw the 5-page decision, knowing that they were citing Rappler, and ABS-CBN, and Inquirer, medyo napayapa na po kami because in law, all these newspaper accounts are mere hearsay," said Roque, a former human rights lawyer.

(We calmed down a bit because in law, all these newspaper accounts are mere hearsay.)

Duterte "shrugged off the opinion after checking the sources, media sources," Roque said in a press briefing.

"Even if magpatuloy pa ang imbestigasyon, kung puro galing sa media ang gagamiting sources at makakaliwang grupo, hind po ‘yan tatayo... It's no big deal," said the Palace official.

(Even if the investigation continues, if all sources are from the media and left-leaning groups, that will not stand.)

The justice department and an inter-agency body "are now investigating, have been investigating, and will file charges and punish those who are guilty of murder," he added.

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Philippine security forces say they have killed 6,117 suspected drug dealers because they fought back violently, but rights groups say authorities have summarily executed drug suspects.

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Duterte, who in March 2018 cancelled the Philippines' membership of the ICC's founding treaty, will not cooperate with the probe, Roque had said, while rejecting the ICC prosecutor's findings.

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"We will not cooperate because we are no longer a member," Roque told a news conference on Tuesday.

Under the ICC's statute, it has jurisdiction for crimes committed while a country was a member until a year after it sought to withdraw, in this case between 2016 and 2019, when the Philippines pullback became official.

Then ICC chief prosecutor Fatou Bensouda had said last December there were reasonable grounds to believe crimes against humanity had been committed during Duterte's bloody anti-narcotics crackdown, whose death toll has stirred international outrage.

Normita Lopez, whose son was a victim of the anti-drugs campaign, said she could not contain her happiness when she learned about the ICC prosecutor's request to open a full investigation into the killings.

"I am happy because I realized that justice never sleeps," said Lopez, 56, who is among the many complainants to the ICC calling for Duterte's international indictment over thousands of alleged extra-judicial killings.

"God is not sleeping, He always finds a way," she said. Her 23-year-old son was killed in May 2017 for allegedly resisting arrest during a sting operation.

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Despite concerns from the international community about the crackdown on drugs, Duterte remains popular at home and many Filipinos back his tough stance on crime.

His single 6-year term as president will end in June next year, and political analysts say he would want an ally to win the presidency to protect him from potential legal challenges and political vendettas once he loses immunity out of office.

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"We do not need foreigners to investigate killings in the drug war because the legal system is working in the Philippines," Roque had said, adding he believed launching a formal probe was "legally erroneous and politically motivated."

Roque said police used appropriate force and there was "no intention to target and kill civilians."

But Randy delos Santos, uncle of high school student Kian delos Santos who was killed by police officers in August 2017, said he refused to believe government claims that the victims had fought back.

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He said he hoped reports on his nephew's death, which form part of the ICC report, will pave the way for other families of drug war victims to secure justice.

"I welcome the ICC (prosecutor's move). There are many who died (in the drugs war). I feel the pain of other families," delos Santos told Reuters.

— With reports from Jamaine Punzalan, ABS-CBN News; Reuters