The International Criminal Court (ICC) has opened a preliminary investigation into Philippines President Rodrigo Duterte’s “war on drugs.”

The campaign has led to nearly 4,000 deaths since its beginning in July 2016.

The ICC will look into whether crimes against humanity have been committed and whether the Hague-based court has the authority to eventually bring suspects to trial.

A Philippine lawyer sent the ICC a complaint against Duterte and at least 11 senior officials in April 2017, saying crimes against humanity were committed "repeatedly, unchangingly and continuously" and killing drug suspects and other criminals had become "best practice".

The investigation is just the first step in a process that could lead to charges and a trial, though since its inception in 2002 only nine other cases out of 12,000 complaints have gone to trial at the ICC.

Most of the dead were poor Filipinos living in urban areas and human rights group fear the actual number of dead could be far higher than reported.

"While some of such killings have reportedly occurred in the context of clashes between or within gangs, it is alleged that many of the reported incidents involved extra-judicial killings in the course of police anti-drug operations,” said ICC prosecutor Fatou Bensou.

Henry Roque, spokesperson for the bombastic and controversial Mr Duterte, said that the case was a “waste of the court’s time and resources” since the prosecutor’s statement does not actually name any suspects.

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Mr Roque also said that the Philippines leader is more than willing to “be in court and put the prosecutor on the stand” because he is “tired” of the accusations.

“Hypocritical” and “useless” are words Mr Duterte has used to describe the ICC in the past, practically daring the world body to bring him to the Netherlands.

He has threatened to withdraw the Philippines from the court and claimed the lawyers involved were “rotten” and that human rights groups who have claimed he has refused to crackdown on police corruption are just biased against him.

Philippines Senator Antonio Trillanes and Congressman Gary Alejano - dubbed “domestic enemies of the state” by Mr Roque - have said to the court that Mr Duterte has given what amount to “shoot-to-kill” orders in the public.

The most controversial quotes from Rodrigo Duterte Show all 9 1 /9 The most controversial quotes from Rodrigo Duterte The most controversial quotes from Rodrigo Duterte On killing drug addicts “These sons of whores are destroying our children. I warn you, don’t go into that, even if you’re a policeman, because I will really kill you. If you know of any addicts, go ahead and kill them yourself as getting their parents to do it would be too painful” The most controversial quotes from Rodrigo Duterte Message to China “I will go there on my own with a Jet Ski, bringing along with me a [Phillipino] flag and a pole, and once I disembark, I will plant the flag on the runway and tell the Chinese authorities, ‘Kill me’ ” AP The most controversial quotes from Rodrigo Duterte Christmas message to law-breakers “If you do not want to stop, and just continue committing crimes, then this would be your last Merry Christmas” AP The most controversial quotes from Rodrigo Duterte On sex life “I was separated from my wife. I’m not impotent. What am I supposed to do? Let this hang forever? When I take Viagra, it stands up” AFP/Getty Images The most controversial quotes from Rodrigo Duterte On the drugs trade “None of my children are into illegal drugs. But my order is, even if it is a member of my family, ‘kill him'" AP The most controversial quotes from Rodrigo Duterte Insulting the Pope “We were affected by the traffic. It took us five hours. I asked why, they said it was closed. I asked who is coming. They answered, the pope. I wanted to call him: ‘Pope, son of a wh**e, go home. Do not visit us again’” AFP/Getty Images The most controversial quotes from Rodrigo Duterte Joke about rape “I saw her face and I thought, 'What a pity... they raped her, they all lined up. I was mad she was raped but she was so beautiful. I thought, the mayor should have been first” AFP/Getty The most controversial quotes from Rodrigo Duterte Insulting Barack Obama "Mr Obama should be respectful and refrain from throwing questions at me about the killings, or son of a bitch, I will swear at you in that forum" REUTERS The most controversial quotes from Rodrigo Duterte On Abu Sayyaf Islamic militants "If I have to face them, you know I can eat humans. I will really open up your body. Just give me vinegar and salt, and I will eat you. If you annoy me to the fullest... I will eat you alive. Raw" EPA

"He will assert the legality of the war against drugs as a valid exercise of sovereign powers and, therefore, the element required for a crime against humanity is lacking," Mr Roque said.

Just days before a November 2017 visit from US President Donald Trump, Mr Duterte has said he killed someone when he was just 16 years old.

Speaking to the Filipino community in Da Nang, Vietnam, to promote his drug war policies, he claimed: “At the age of 16, I already killed someone. A real person, a rumble, a stabbing. I was just 16 years old. It was just over a look. How much more now that I am President?”

Mr Duterte won last year’s presidential elections after promising to eradicate illegal drugs with an unprecedented crackdown that would see up to 100,000 people killed. Since he took office 16 months ago, police say they have killed 3,967 people. Another 2,290 people were murdered in drug-related crimes, while thousands of other deaths remain unsolved, according to government data.

Aides, however, have said the 72-year-old leader is prone to “hyperbole” but the stories of his past violence have resurfaced multiple times.

In 2016, Mr Duterte said that he would be “happy to slaughter” millions of drug addicts in the island nation and referred to former President Barack Obama as a “son of a whore” when the US leader criticised the policy.

Mr Trump and Mr Duterte also had an eyebrow-raising phone call in April of this year, when he told the Philippines leader that he thought he was doing a “great job” on the drug war.