"Since 2016, I have closely followed the situations" in both the Philippines and Venezuela, prosecutor Fatou Bensouda said in a statement.
After "a careful, independent and impartial review... I have decided to open a preliminary examination into each situation."
In the Philippines, her office would "analyse crimes allegedly committed... since at least 1 July 2016, in the context of the 'war on drugs' campaign" launched by Philippine President Rodrigo Duterte.
Many were allegedly "extra-judicial killings" carried out during "police anti-drug operations."
The ICC was opened in 2002 to prosecute the world's worst crimes.
The Philippines probe will be its first preliminary examination in a Southeast Asian nation.
In Venezuela, Bensouda said her office would probe crimes allegedly committed during demonstrations and political unrest since April 2017 against the government of President Nicolas Maduro.
"In particular, it has been alleged that state security forces frequently used excessive force to disperse and put down demonstrations, and arrested and detained thousands of actual or perceived members of the opposition," she said.
Manila had been informed earlier that the ICC probe was to be opened and Duterte denied all charges of mass murder and crimes against humanity, his spokesman said.
"We view of course this decision of the prosecutor as a waste of time and resources," spokesman Harry Roque said, adding that Duterte merely employed "lawful use of force" against threats to the state and its citizens.
He has since overseen a crackdown that has left nearly 4,000 drug suspects dead at the hands of the police. The authorities are also investigating more than 2,000 other cases of "drug-related" killings by unknown suspects.
Rights groups have put the total number of drug war deaths as at least twice the official figure, many of them committed by shadowy vigilantes.
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