The Philippine government said Friday it intended to appeal an International Criminal Court decision to reopen an inquiry into Manila's brutal anti-drug campaign, which left thousands dead.

Former president Rodrigo Duterte, who initiated the drug war, pulled the Philippines out of the ICC in 2019, a year after the Hague-based tribunal began a preliminary probe into the crackdown.

The ICC launched a formal inquiry in September 2021, only to suspend it two months later after Manila said it was re-examining several hundred cases of drug operations that led to deaths at the hands of police, hitmen and vigilantes. The ICC prosecutor later asked to reopen the inquiry in June 2022.

Announcing the probe's resumption on Thursday, the ICC said its pre-trial chamber was "not satisfied that the Philippines is undertaking relevant investigations that would warrant a deferral of the court's investigations".

Menardo Guevarra, the chief lawyer for current Philippine President Ferdinand Marcos' government, told AFP: "It is our intention to exhaust our legal remedies, more particularly elevating the matter to the ICC appeals chamber."

Officially, 6,181 people were killed in Duterte's "war on drugs", which began in 2016, but rights groups say that up to 30,000 may have been killed, some innocent victims, and that corruption was rife among security forces that acted with impunity.

President Marcos, elected in a landslide last year, has vowed to continue the drug war but with a focus on prevention and rehabilitation. He has, so far, ruled out reversing Duterte's decision to pull the Philippines out of the ICC.

"We wish to emphasise that our own domestic investigative and judicial processes should take precedence" over the ICC, said Guevarra, the Philippine solicitor general.

"We can show that despite structural and resource limitations in our legal system, it is still a well-functioning system that yields positive results in its own time," he added.

Rights groups, however, welcomed the ICC announcement, and allege the killings are continuing under Marcos.

National Union of People's Lawyers chairman Edre Olalia told AFP: "This is indeed very welcome news coming as it does in the midst of continuing impunity, selective memory and orchestrated denial by the past and present governments."

The group represents some of the dead suspects' families in a handful of cases being tried in Philippine courts against police officers.

Olalia said the ICC announcement "validates" the assertions of the slain suspects' kin that "there are no adequate and effective measures to achieve concrete justice for them on the ground... despite official claims to the contrary".

Only three police officers have been convicted of unlawful drug war killings, while another police officer was jailed in November last year for planting evidence and torturing two teenagers killed at the height of the crackdown.

"The ICC investigation in the Philippines is the only credible avenue for justice for the victims and their families of former President Rodrigo Duterte's murderous 'war on drugs'," Human Rights Watch Asia deputy director Phil Robertson said in a statement.

Renato Reyes, a senior leader of the left-wing group Bagong Alyansang Makabayan (New Nationalist Alliance), urged Marcos in a statement to cooperate with the ICC probe "so that justice can be rendered to the thousands of victims of Duterte's failed drug war".