“We are going to set up a separate tribunal to show these people are not untouchable,” Zelensky told a news conference. “We need justice.”

The International Criminal Court, a permanent war crimes court based in The Hague, in March issued an arrest warrant for Putin for suspected deportation of children from Ukraine, which would be a war crime.

But the ICC does not have jurisdiction over the crime of aggression in Ukraine. An act of aggression is defined by the United Nations as the “invasion or attack by the armed forces of a state (on) the territory of another state, or any military occupation”.

The European Commission, among others, has said it supports the creation of a separate international centre for the prosecution of the crime of aggression in Ukraine, that would be set up in The Hague.

“We all want to see a different Vladimir here in The Hague, the one who deserves to be sanctioned for his criminal actions here, in the capital of international law,” Zelensky said in a speech earlier in the day, referring to Putin.

“I’m sure we will see that happen when we win, and we will win,” he said.

Major legal and practical questions remain around how a new court to judge aggression would be legitimised, either by a group of countries supporting it or with approval from the U.N. General Assembly.

Russia is not a member of the ICC and already rejects its jurisdiction. It denies committing atrocities during its conflict with Ukraine, which it terms a “special operation” to “demilitarise” its neighbour.

Earlier in the day, as he left the ICC after a visit of just under an hour, Zelensky, dressed in his trademark khaki, waved at a Ukrainian family standing outside the ICC building as they shouted “Slava Ukraini” – or Glory to Ukraine.

The Netherlands has been a strong supporter of Ukraine, with Dutch Prime Minister Mark Rutte in February saying he did not rule out any kind of military support for Kyiv as long as it did not bring NATO into conflict with Russia.

Pledging “unwavering support,” Rutte said there were “no taboos” on sending F-16 fighter jets to Ukraine, and that discussions were underway with other countries on the matter, before adding: “We are not there yet.”

Russia has stepped up attacks as Ukraine prepares for a counteroffensive to try to retake Russian-occupied land in the south and east. Russian shelling in the frontline southern region of Kherson killed at least 23 civilians on Wednesday.

By Stephanie van den Berg and Toby Sterling

(Reporting by Bart Meijer, Stephanie Van Den Berg, Toby Sterling; Writing by Ingrid Melander; Editing by Nick Macfie, Alexandra Hudson)