An international investigation office seen as a "truly historic" first step towards a possible trial of Russia's leadership opened in The Hague on Monday to probe a crime of aggression against Ukraine.

The centre, grouping prosecutors from the EU, Ukraine, the United States and the International Criminal Court (ICC), will gather evidence against Kremlin and Russian military chiefs over the invasion launched in February 2022.

Its aim is to plug a legal gap left by the fact that the ICC currently has no mandate to prosecute aggression -- what Ukraine has called the "supreme international crime" of launching a war against another country.

"We are gathered here on the occasion of a truly historic moment -- I would say an epoch-defining moment," Ukraine's Prosecutor General Andriy Kostin told a news conference in The Hague.

Kostin said a special tribunal was now "inevitable", describing the centre as a "clear signal that the world is united and unwavering on the path to holding the Russian regime accountable for all its crimes".

"If the crime of aggression had not been committed, there would be no other 93,000 incidents of war crimes," he said.

- 'Significant day' -

The new International Centre for the Prosecution of the Crime of Aggression Against Ukraine (ICPA) is based at the headquarters of the EU judicial office Eurojust in the Dutch city. 

It will gather evidence, including photos and communications intercepts, both for national investigations underway by five European countries including Ukraine and also for any future tribunal.

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

The Hague-based ICC has already issued a war crimes arrest warrant for Russian President Vladimir Putin over the alleged forced deportation of Ukrainian children.

But the court's rules do not allow it to prosecute Russia over the "leadership" crime of aggression, which the ICC's founding Rome Statute defines as the attack of one state on another in breach of the UN charter.

ICC prosecutor Karim Khan hailed the opening of the centre as a "significant day", saying it was the "first time that I can recall" that war crimes evidence was already being gathered in the middle of a conflict. 

- 'Gross violation' -

The EU announced the creation of the ICPA in February and the United States said 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. 

"It will not be the last," he told the news conference.

Polite added that Washington was "proud to stand with our European partners" in prosecuting "Russia's unlawful war of aggression against the people of Ukraine". 

EU Justice Commissioner Didier Reynders said the new office showed that 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 said.

But crucial questions remain about any possible special tribunal, including how it would work, where it would be based, 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.

Eurojust chief Ladislav Hamran said the key for now was to collect evidence rather than worry about a future tribunal.

"As far as investigation of the crime of aggression goes, it's important that we start now," he said 

dk/js