The Hague (AFP) - An international office to probe Russia for the war crime of aggression opened in The Hague on Monday in what Ukraine called a “truly historic” first step towards a tribunal for Moscow’s leadership.

The centre will investigate and gather evidence for any future trial that could bring Kremlin and Russian military figures to justice for invading their pro-Western neighbour.

Its aim is to plug a legal gap left by the fact that the International Criminal Court (ICC) currently has no mandate to prosecute aggression – the core war crime of launching a war against another country.

The new International Centre for the Prosecution of the Crime of Aggression Against Ukraine (ICPA) features prosecutors from Kyiv, the European Union, the United States and the ICC.

Speaking as the centre opened, Ukraine Prosecutor General Andriy Kostin said a special tribunal for the Russian leadership was now “inevitable”.

“We are gathered here on the occasion of a truly historic moment – I would say an epoch-defining moment,” Andriy told a news conference at the EU judicial office Eurojust, where the ICPA is based.

Kostin said the opening of the centre was a “clear signal that the world is united and unwavering on the path to holding the Russian regime accountable for all its crimes.”

He added that the “crime of aggression is an original sin, the commission of which opened the floodgate for 100,000 other international crimes.”

Kyiv has been pushing for a special tribunal since the discovery of hundreds of bodies after Russian troops withdrew from the town of Bucha near the Ukrainian capital in April 2022.

- ‘Unlawful war’ -

International support has grown steadily, and the European Commission then announced the creation of the ICPA in February.

The United States then announced that it would join last month – despite the fact that, like Russia, it is not a member of the ICC.

US Assistant Attorney General Kenneth Polite said justice officials have now handed over the first tranche of evidence to the centre.

Polite told the news conference that Washington was “proud to stand with our European partners” in prosecuting “Russia’s unlawful war of aggression against the people of Ukraine.”

The United States also backed a special tribunal for aggression, he added.

EU Justice Commissioner Didier Reynders said Monday’s launch showed Kyiv’s allies would “stand with Ukraine for as long as it takes.”

“We cannot tolerate the gross violation of the prohibition of the use of force,” he told the news conference.

Calls for a special tribunal on Ukraine have mounted because of the inability to prosecute aggression by the ICC, a war crimes court which is also based in The Hague.

The ICC is probing more specific war crimes and crimes against humanity in Ukraine, and issued an arrest warrant for Russian President Vladimir Putin in March over alleged child deportations.

Fundamental questions remain over how a special tribunal would work, when it could be created and who would support it.

The most likely option appears to be a hybrid court under Ukrainian law with Ukrainian and foreign judges.

But Eurojust chief Ladislav Hamran said it was “not important at this stage… where the trial will happen”.

“As far as investigation of the crime of aggression goes, it’s important that we start now,” he said