The International Criminal Court (ICC) prosecutor has said he will “immediately proceed” with an investigation into alleged war crimes in Ukraine dating back to 2013, when protests erupted against a Russia-friendly government in Kyiv.

Karim A A Khan said late on Wednesday that the probe was starting after 39 countries that are parties to the Rome Statute that established the court, including Canada and France, petitioned the ICC to begin an investigation.

“These referrals enable my Office to proceed with opening an investigation into the Situation in Ukraine from 21 November 2013 onwards,” Khan said in a statement, adding that its scope would encompass “any past and present allegations of war crimes, crimes against humanity or genocide committed on any part of the territory of Ukraine by any person”.

The announcement comes a week after Russia launched an all-out attack on Ukraine, drawing condemnation and sanctions from major world powers.

The ICC prosecutor had made public his intention to launch an investigation on Monday, saying that there was a “reasonable basis” to believe that war crimes have occurred during the conflict.

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“I have notified the ICC Presidency a few moments ago of my decision to immediately proceed with active investigations in the Situation. Our work in the collection of evidence has now commenced,” Khan said in Wednesday’s statement.

Established in 2002, the Hague-based court investigates and prosecutes genocide, war crimes, and crimes against humanity.

Last week, Khan warned the warring parties that his office had jurisdiction over Ukraine because the Ukrainian government accepted the ICC’s mandate in 2015, despite the country initially not being a party to the Rome Statute.

In the days since Russian troops began their offensive last Thursday, the violence has intensified in several major Ukrainian cities, prompting hundreds of thousands of people to flee the country in search of safety.

Russian forces entered the southern port city of Kherson late on Wednesday, its mayor was cited as saying by the Reuters news agency and The New York Times.

But the Russian military’s advance on the capital Kyiv “remains stalled”, the Pentagon said earlier in the day, as Moscow’s forces regroup and face logistical challenges and Ukrainian resistance.

The Stat