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The UK on Friday vowed to help Ukraine "pursue criminal accountability for Russia's illegal invasion", as international support grows for a special tribunal to prosecute Russian leaders.

Foreign Secretary James Cleverly branded Moscow's renewed military assault on its neighbour, launched last February, "an outrageous violation of the rules-based international order".

He said London had accepted an invitation from Kyiv to join "a core group of like-minded partners" seeking legal accountability, with a new "hybrid" tribunal among the potential options to be assessed.

"Alongside other international partners invited by Ukraine, the UK will shape thinking on how to ensure criminal accountability for Russia's aggression against Ukraine," his department said.

"This includes assessing the feasibility of a new 'hybrid' tribunal".

A specialised court could be integrated into Ukraine's national justice system but have foreign elements, such as international prosecutors and judges and foreign funding.

The idea first emerged late last year and won the backing of German Foreign Minister Anna Baerbock on Monday during a visit to The Hague, which hosts the International Criminal Court (ICC).

It would get around the fact that the ICC cannot prosecute Russia for the "leadership" crime of aggression, because Russia does not recognise the court.

The ICC has nonetheless launched an investigation into war crimes and crimes against humanity in Ukraine, because Kyiv has accepted its jurisdiction.

But there are growing Western calls for a better mechanism to target Russia's civilian and military leadership for ordering and overseeing the invasion.

"The atrocities we've witnessed in Ukraine are diabolical (and) must not go unpunished," said Cleverly.

"That's why the UK has accepted Ukraine's invitation to join this coalition, bringing our legal expertise to the table to explore options to ensure Russia's leaders are held to account fully".

However, there are major hurdles before any proposed special court could even be created, let alone put Russian leaders on trial.

ICC prosecutor Karim Khan, who held talks with Baerbock on Monday, has opposed the creation of a special tribunal for Ukraine, fearing it could undermine his own probes into Russia's war.

Meanwhile, broad international support would be seen as crucial to its formation, before the thorny issue of how to bring any suspects before it is tackled.

Russia has said any Ukraine tribunal would lack legitimacy, and would refuse to extradite suspects.