Philippine President Rodrigo Duterte won't cooperate with an International Criminal Court investigation into the country's drug killings, his representative said Tuesday.

The court's Office of the Prosecutor applied for authorization to investigate alleged crimes against humanity committed during Duterte's campaign against illegal drugs.

"We will not cooperate because we are no longer a member," Duterte's presidential representative Harry Roque told a news conference.

Court chief prosecutor Fatou Bensouda, whose term ends Tuesday, said an initial investigation started in February 2018 determined "there is a reasonable basis to believe that crimes against humanity of murder have been committed" in the Philippines between 2016 and 2019, Al Jazeera and others said.

CNN Philippines shared a tweet of Bensouda's investigation:

The president canceled the southeast Asian nation's membership of the court in March 2018. Bensouda also seeks to investigate killings in Davao City from 2011 to 2016.

Duterte was mayor for more than two decades. His daughter Sara "Inday" Duterte currently holds that office.

Various human rights groups documented more than 1,400 killings - allegedly by a group popularly known as the Davao Death Squad - between 1998 and 2016.

In an address recorded this week before news of Bensouda's investigation was made public, Duterte called on human rights groups to examine closely his war on drugs. "You'd notice there are really people who die almost daily because they fought back," the president said, warning drug users: "Do not destroy the country. I will kill you."

Duterte is willing to go to prison if found guilty in the Philippines. "If the court says I should go to jail, I will go to jail. That's no problem," he said in a televised speech in December.

Former senator Antonio Trillanes IV, a vocal critic of the Duterte administration, said "the long arm of the law will soon catch up with Duterte and his accomplices."

"It was all worth the wait, notwithstanding the long nights of grief and grim days of fear. Indeed, there will be justice somewhere, somehow," The Vera Files quoted National Union of People's Lawyers President Edre U. Olalia saying.

Malacañang Palace hasn't issued a statement on the court's announcement.

Meanwhile, international lawyer Priya Pillai told Rappler in an interview the Philippines' withdrawal will make it difficult for the court to investigate.

"Withdrawal will make any investigation more difficult as there will be no guarantee of actual cooperation," Pillai said.

Davao had the highest murder rate, the second-highest rape rate and the fourth-highest number of index crimes in the Philippines, according to official police data for 2010 to 2015.

As of April 2015, Davao City improved to fifth in the ranking of the World's Safest Cities.

Duterte took office aged 71 in June 2016 - making him the oldest Philippine president.

Based in The Hague, the court was established to investigate and prosecute genocide, crimes against humanity and war crimes.