It looks like Putin and his circle are so far safe from international justice, at least while they remain in power. By invading the sovereign nation of Ukraine, under non-justifiable pretexts (de-Nazification and defence of pro-Russian population suffering from genocide in Donbas region), Russia has breached the very first article of the UN Chart:

To maintain international peace and security, and to that end: to take effective collective measures for the prevention and removal of threats to the peace, and for the suppression of acts of aggression or other breaches of the peace, and to bring about by peaceful means, and in conformity with the principles of justice and international law, adjustment or settlement of international disputes or situations which might lead to a breach of the peace.

And therefore, defied international law, Vladimir Putin is now considered a war criminal. The campaign Justice for Ukraine calls for a tribunal inspired on the legal framework created to prosecute criminals from the Second World War in the Nuremberg trials. This tribunal would judge Putin and his closer allies, under the (international) crime of aggression; however, it has never before happened that an active head of state is extradited to be tried by an international tribunal. 

Among those who have signed the petition for the pursuit of the campaign Justice for Ukraine, senior figures from the legal community have joined leading academics and politicians. To name a handful of people: Ukraine foreign minister Dmytro Koleba, former prosecutor for the Nuremberg Military Tribunal Benjamin Ferencz, former president of the European Court of Human Rights Sir Nicolas Bratza, former UK prime ministers Sir John Major and Gordon Brown, Mark Ellis executive director of the International Bar Association, Yves Fortier former president of the United Nations' security council, Covington & Burling senior counsel Peter Trooboff, and Delhi-based international law consultant Anjolie Singh.

Why is there a need for a separate tribunal on this occasion? The possibilities for the International Criminal Court ICC to prosecute Russia for its war crimes have been de-estimated by the campaign, as it must be the UN Security Council who refers the crime of aggression, which will never happen taking into account Russia’s veto power at the UN, and Russia doesn’t even recognize the ICC law as binding. Whereas the International Court of Justice ICJ, has already, on the past 16th of March, mandated Russia the cease of any military offensive; nonetheless, this tribunal cannot oblige to follow its mandates; it is still a first victory for Ukraine.

The mission for this special tribunal appears improbable but not impossible, first many things have to fit. In Ukraine, the initial step (collection of evidence) is already underway. We take the examples of when Russians bombed the Donetsk Regional Drama Theater in Mariupol, the beleaguered city's largest air-raid shelter that may hold as many as 1,300 people and had the word Deti ("Children") painted in huge Cyrillic characters on the Exterior; however, it is difficult to prove that Putin personally ordered the bombing of hospitals or schools. Then evidence must then be considered in court. Appealing to the United Nations Convention on the Prevention and Punishment of the Crime of Genocide, Ukraine asked the ICJ to reject the Russian argument that “its invasion was justified because Ukraine had committed genocide in the secessionist Luhansk and Donetsk regions”. The court has agreed with Ukraine by 13 votes in favour and two against (the exceptions, the Russian judge and the Chinese judge).