President Rodrigo Duterte said on Wednesday he was pulling the Philippines out of the treaty underpinning the International Criminal Court, which is examining his deadly drug war.

"I therefore declare and forthwith give notice... that the Philippines is withdrawing its ratification of the Rome Statute effective immediately," Duterte said in a statement.

The Hague-based ICC announced last month it was launching a "preliminary examination" of Duterte's bloody anti-drug crackdown that has drawn international concern.

Police say they have killed nearly 4,000 drug suspects as part of the campaign, while rights groups claim the toll is around three times the numbers given by authorities.

The outspoken Philippine leader, who is accused of stoking the killings with inflammatory statements, has taken issue over the Philippines becoming the first southeast Asian nation put under a preliminary examination by the ICC prosecutor.

Opened in 2002, the ICC is the world's only permanent war crimes court and aims to prosecute the worst abuses when national courts are unable or unwilling.

The Philippines, under previous President Benigno Aquino, ratified in 2011 the Rome Statute which underpins the ICC, giving the tribunal authority to investigate crimes on its soil.

On Wednesday Duterte, a former lawyer, attacked the ICC's preliminary examination into his anti-narcotics campaign saying it was "unduly and maliciously created".

"It is apparent that the ICC is being utilised as a political tool against the Philippines," he said.

In his statement, Duterte cited "baseless, unprecedented and outrageous attacks on his person as well my administration".

"The acts allegedly committed by me are neither genocide nor war crimes. The deaths occurring in the process of legitimate police operations lacked the intent to kill," he added.

But even as early as October 13, 2016, when Duterte had only been in office less than four months, the ICC prosecutor Fatou Bensouda said in a statement that she was "deeply concerned" over reports of extra-judicial killings of over 3,000 alleged drug users and pushers.

Adding pressure on Manila, in February, the UN Human Rights Council in Geneva raised the country's human rights record with Iceland Foreign Minister Gudlaugur Thor Thordarson calling on the Philippines to accept the visit of a UN Special Rapporteur.

Philippine officials had initially said in February that the country was ready to cooperate but asked for fairness.

Duterte's spokesman Harry Roque also said they would refuse a visit by one such rapporteur Agnes Callamard, who had previously been pressing to investigate the killings.

But Roque has also said the ICC has no jurisdiction over the case because the tribunal was intended as a "court of last resort" and the Philippine courts were fully functioning.