The European Union is deeply shocked by the mass graves containing more than 440 bodies found by Ukrainian authorities in Izium, a city recently liberated from Russian occupation forces, EU High Representative for Foreign and Security Policy Josep Borrell said in a statement.

"We condemn these atrocities in the strongest possible terms," the statement says.

"Russia's war of aggression against Ukraine has been leaving a trail of blood and destruction across Ukraine. Thousands of civilians have been already murdered, many more tortured, harassed, sexually assaulted, kidnapped, or forcibly displaced. This inhuman behaviour by the Russian forces, in total disregard of international humanitarian law and the Geneva conventions, must stop immediately," the statement says.

"Russia, its political leadership, and all those involved in the ongoing violations of international law and international humanitarian law in Ukraine will be held accountable. The EU supports every effort in this regard," Borrell said.