Ukraine has opened 5,600 investigations for alleged war crimes committed in the territory since the beginning of the Russian invasion, Ukrainian Prosecutor General Iryna Venediktova said Sunday.

Speaking to the Sky News television station, the prosecutor called Russian President Vladimir Putin "the leading war criminal of the 21st century," and said she had identified 5,600 cases of alleged war crimes as well as 500 Russian war criminals.

Alluding to the attack targeting the Kramatorsk (eastern) station in which 52 civilians, including five children, were killed in an attack attributed to a Russian missile, Iryna Venediktova said she had "proof" that Russia was behind the attack.

"Absolutely, it's a war crime," he said, pointing out that "these people just wanted to save their lives, they wanted to get out" of the region.

The prosecutor also thanked British Prime Minister Boris Johnson, who visited Kiev on Saturday as part of an unexpected trip where he met with Ukrainian President Volodymyr Zelensky and promised Ukraine new weapons.

It is "really a great support for us," Venediktova said of Johnson's visit, the first by a G7 official to Ukraine since the invasion began.

Last week, Ukrainian President Volodymyr Zelensky announced the creation of a "special mechanism" to "investigate and prosecute all crimes of the occupiers" of the country, adding that it would operate on the basis of "joint work of national and international experts."

"This mechanism will help Ukraine and the world bring to justice those who initiated or participated in any way in this terrible war against the Ukrainian people and crimes against our people," the head of state explained.