PARIS — An international center focused on building cases against top Russian leaders opened on Monday in The Hague, in what supporters described as a first step toward a possible future tribunal to prosecute the most serious war crimes related to Russia’s invasion of Ukraine.
If established, such a tribunal would be designed to address the gravest war crime, the crime of aggression, which legal experts say is by definition a crime by the leadership of a country that initiates a war against another.
“It’s the only crime that goes to the top table,” said Philippe Sands, a prominent international lawyer who first floated the idea of an aggression tribunal.
A special tribunal would fill a gap in international law. While the International Criminal Court in The Hague has a mandate to prosecute aggression, it does not have jurisdiction on the issue as it involves Russia and Ukraine. When drafting the rule, several powerful countries, including the United States, prevailed in insisting that only leaders from I.C.C. member states could be prosecuted for aggression — and only after the country had explicitly granted the court jurisdiction for that crime. Neither Russia nor Ukraine is a member of the court.
While it is not clear whether a tribunal will be created, the new center, the International Center for the Prosecution of the Crime of Aggression, also aims to serve as a common platform to coordinate investigations, avoiding duplication of tasks and helping organize the work of the many outside experts and forensic teams now working in Ukraine.
Investigators, many from Eastern Europe, will work to join data and match evidence to help build cases for use in future trials both in Ukraine and in international courts.
The center is based at Eurojust, the headquarters of the European Union agency for judicial cooperation, and has received initial funding of $9 million (8.3 million euros) from the bloc. Other countries, including Britain and the United States, have also pledged support for the initiative.
Since Russia’s full-scale invasion of Ukraine began more than 16 months ago, Western diplomats and lawyers have been discussing the need for a separate international tribunal with the power to prosecute Russia for the crime of aggression. Aggression is distinct from the offenses of war crimes, crimes against humanity and genocide, for which the I.C.C. can hold individuals accountable.
Advocates of a special tribunal argue that aggression is the paramount crime from which others emanate, and prosecuting that crime most directly implicates the political or military leaders who decide to wage war. And unlike in many war crimes cases, there would be no need to link an official to specific crimes on the ground, which is often a cumbersome process.
Still, governments that support the idea of a tribunal disagree over whether it should be formed under the United Nations or become a specialized independent court focused on Ukraine, with international backing.
The prosecutor of the International Criminal Court in The Hague is in the middle of an investigation of crimes against humanity and war crimes committed in Ukraine. The court has issued arrest warrants for President Vladimir V. Putin of Russia and another top Russian official over the abductions of Ukrainian children during the conflict. But the Kremlin has denied committing crimes, and the likelihood of a trial remains slim.
Overwhelmed courts in Ukraine have already tried and convicted some Russian soldiers for war crimes, but have tens of thousands of cases waiting.