MANILA (REUTERS) - The Philippine Supreme Court has thrown out a petition that sought to invalidate President Rodrigo Duterte's unilateral withdrawal from the International Criminal Court (ICC), which has been examining allegations of atrocities in his bloody war on drugs.

Mr Duterte in March 2018 cancelled the Philippines' membership of the ICC's founding treaty just weeks after the ICC's prosecutor announced that a preliminary examination was under way into thousands of killings in his war on drugs, which he said was prejudiced against him.

But six minority senators asked the Supreme Court to invalidate Mr Duterte's decision, which took effect a year later, arguing that it was illegal and done without Senate approval, which is needed before entering into treaties.

The Supreme Court said in a statement that judges unanimously dismissed the legal challenge as "moot and academic".

"The court also noted that the judiciary has enough powers to protect human rights, contrary to speculation raised by the petitioners," it added.

The ICC is a court of last resort that can exercise jurisdiction if states are unable or unwilling to investigate crimes, which Mr Duterte's office has repeatedly said was not the case in the Philippines.

Despite the withdrawal, the ICC has been receiving complaints and testimony from activists calling for Mr Duterte's international indictment over thousands of alleged extrajudicial killings during his campaign against narcotics, which took place while the Philippines was an ICC member.

Human rights groups accuse Mr Duterte of inciting deadly violence and say police have murdered unarmed suspects and staged crime scenes on a massive scale. The police reject that and Mr Duterte insists that he told police to kill only in self-defence.

Mr Salvador Panelo, Mr Duterte's chief legal counsel, said the Supreme Court's decision "puts to rest the debate on the authority of the president to withdraw from treaties and international agreements".