"While continuing to support the International Criminal Court, we are proposing to set up a specialized court backed by the United Nations to investigate and prosecute Russia's crime of aggression," said European Union chief Ursula von der Leyen.

Ukraine has long been pushing for such a specialized tribunal, with its president, Volodymyr Zelensky, saying Russia must be brought to justice for its "atrocities".

But the initiative faces formidable legal and political obstacles.

The problem Ukraine and its Western allies butt up against is that the International Criminal Court (ICC) does not have jurisdiction over Russia's "crimes of aggression" -- its invasion and war in Ukraine -- because Moscow is not a signatory to the court's treaty.

The only way the ICC can be called in to judge Russia's war is through a decision by the UN Security Council -- something that is impossible because Russia, with its permanent seat on the council, would veto it.

To get around that, von der Leyen is proposing to have a court set up in an EU country that could tackle Russia specifically on the crime of aggression, while leaving war crimes and crimes against humanity to the ICC.

The Netherlands, which already hosts the ICC in The Hague, has indicated its willingness to establish the mooted new court on its territory.

A proposal text circulated by von der Leyen's European Commission noted that 14 EU member states had already opened investigations into acts carried out in Ukraine "based either on personal or universal jurisdiction".

It said that, where it came to crimes of aggression, "an alternative investigation mechanism could be considered" where EU countries' interests or citizens were affected.

Commission officials said that path would run alongside the ICC -- to which "the EU will continue to give its full support" -- and would need UN support.

That would involve a procedural gambit where the proposal would submitted to the UN Security Council -- where Russia is sure to shoot it down -- and then to the wider UN General Assembly of all UN member states, where it stands a chance of passing.

In a separate "options paper" also circulated Wednesday, the commission was exploring ways to tap Russian assets frozen under sanctions to support the reconstruction of Ukraine.

Von der Leyen has said the aim was to confiscate the assets seized so far -- which the commission says amounts to around 19 billion euros ($20 billion) in the EU.

But commission officials said legal barriers meant it was more likely only cash proceeds generated from management of those frozen assets could be taken, not the assets themselves.

After any future peace agreement to end the war in Ukraine, "eventually, the assets themselves would need to be returned to the owners," one official said.