Q&A.. Can President Vladimir Putin be prosecuted?

Ukrainian President Volodymyr Zelensky reiterated on Thursday from The Hague his call for the establishment of a special international tribunal to look into what he called the "crime of aggression" in reference to the Russian war on his country.

Can Russian President Vladimir Putin really be prosecuted for the war he unleashed on Ukraine?

Why this call?

The International Criminal Court (ICC) in March issued an arrest warrant for the Russian president for the war crime of "unlawful deportation" of Ukrainian children in the conflict, but the court, set up in 2002 to try the world's worst atrocities, cannot prosecute a country's leaders for aggression if it is not a signatory to the Rome Statute, which is not the case with Russia.

"Impunity is the key to aggression," Zelenskiy said, adding: "There must also be a responsibility to deter future perpetrators."

The "crime of aggression" is similar to the concept of "crime against peace" used in the Nuremberg and Tokyo trials at the end of World War II.

For the ICC, it is a charge that goes as far as acknowledging "the use of armed force by a State against the sovereignty, territorial integrity or political independence of another State.

What does Zelensky want?

Zelenskiy wants a court with "full power" to try the "crime of aggression", which he described as the "beginning of evil" and to "correct the gaps that unfortunately exist in the law," but forming such a court could be difficult.

Overwhelming support at the UN General Assembly could be a potential way to reach such a court, but Moscow would use its permanent seat on the UN Security Council to veto any council resolution supporting it.

Support may also be limited for a court that includes only European countries or a regional organ such as the European Union, experts say.

Stephen Raab, a former U.S. envoy for war crimes, said: "What is most important to achieve this is for countries outside Europe to recognize that they have an interest in preserving the sovereignty and territorial integrity of all states."

He told a news conference in The Hague in February that it was also important "to set a precedent that could be used later in the event of a similar attack against another country."

Are there other options?

Fearing that the idea of creating a special court for Ukraine would "fail," ICC prosecutor Karim Khan suggested that member states find ways to allow the court to prosecute a non-member state for the crime of aggression.

International lawyer Priya Pillay said the ICC could prosecute crimes of aggression "if its basic status is modified" but it was "unlikely" that it would be retroactive.

Raab went on to assert that until a possible amendment to the text was reached, the war would be "long over".

Many of Ukraine's allies, including the United States, have declared their support for the creation of a hybrid court to try "Russian aggression" with funding and international personnel that would be "rooted in the Ukrainian judicial system," but Ukraine "will not accept either a hybrid peace or a hybrid court," Zelensky asserts.

Will Putin be arrested one day?

If Ukraine's president has expressed his conviction that Putin is a "master" in The Hague, experts have warned that the arrest of senior Russian officials will pose a major challenge.

Cecil Rose, an expert on public international law, said "if there is no regime change in Russia", Putin is unlikely to be prosecuted, but history has seen several high-profile figures reach the dock against all expectations.

"The same question was asked for the former Yugoslavia when Slobodan Milosevic was charged," former Serbian president who died of a heart attack in 2006 in the ICTY detention centre, was charged.