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Vladimir Putin has been accused of committing war crimes after Ukraine reported more than 2,000 civilian deaths since Russia's full-scale invasion.
Following days of intensive bombardment, there are fears the Russian president will unleash even greater force against Kyiv and other Ukraine cities in the coming days.
Defence Secretary Ben Wallace has warned Mr Putin will seek to "pummel" cities in tactics reminiscent of medieval siege warfare, including a plan to "carpet-bomb cities indiscriminately".
Following worldwide condemnation of Russia's actions, the chief prosecutor of the International Criminal Court (ICC) has said he plans to open an investigation into events in Ukraine.
Could Mr Putin face trial over Russia's military action and what evidence is there of alleged war crimes being committed in Ukraine?
What is a war crime?
According to the United Nations, the term "war crimes" refers to serious breaches of international humanitarian law committed against civilians or "enemy combatants" during an armed conflict.
Rules during conflicts were agreed after the Second World War in the 1949 Geneva Conventions, which were signed by all members of the UN - including Russia.
A list of war crimes set out by the Rome Statute of the ICC includes "intentionally directing attacks... against individual civilians not taking direct part in hostilities".
It also includes "attacking or bombarding, by whatever means, towns, villages, dwellings or buildings which are undefended and which are not military objectives".
Is there evidence of war crimes committed by Russia in Ukraine?
Wayne Jordash QC, a British humanitarian lawyer working in Ukraine, said there was a "growing list of war crimes" being committed by Russia.
He told Sky News that this appeared partly to be the result of "Russia's frustration with the way their invasion is going".
Mr Jordash said: "We went from a conflict where the Russians were aiming at military targets to one where they are slowly not caring what they're hitting, or they are deliberately aiming at civilian infrastructure in order to scare and dominate the civilian population.
"We're certainly talking about war crimes in every town and every city that has been attacked.
"It's a number that's growing day-by-day."
Mr Jordash said prosecutors in Ukraine, humanitarian lawyers, investigative journalists and civilian activists were gathering evidence of alleged war crimes.
Amnesty International highlighted one attack involving the use of widely-banned cluster munitions on a pre-school in northeastern Ukraine, which it said "may constitute a war crime".
Three people were killed, including a child, when the explosives hit the nursery and kindergarten in the town of Okhtyrka on Friday morning as locals took shelter inside, Amnesty said.
The attack appears to have been carried out by Russian forces which were operating nearby and which have a record of using cluster munitions in populated areas, it added.
Amnesty said Russia's invasion of Ukraine is a clear violation of the United Nations Charter and an act of aggression that is a crime under international law.
Meanwhile, Mr Jordash believes Mr Putin is responsible for a "myriad" of war crimes going back further, when Russia annexed Crimea in 2014 and separatists took control of two territories in eastern Ukraine.
He said: "Since then, Putin has been engaged in widespread, systematic crimes against humanity in those locations."
Could Putin stand trial for war crimes?
The chief prosecutor of the ICC, Karim Khan, has said he wants to open an investigation "as rapidly as possible" into potential war crimes and crimes against humanity in Ukraine.
Russia and Ukraine are not among the court's 123 member states but Ukraine has accepted its jurisdiction, which means Mr Khan can investigate.
Mr Jordash told Sky News that any prosecution of Mr Putin for war crimes would have to prove he was responsible for them.
"War crimes and crimes against humanity may occur without the head of the state being responsible," he said.
"However Putin makes it very clear in the way he behaves, and what he says, that he is in control of the state.
"I think there's a very clear link between what Putin says and does and the crimes on the ground."
What are the potential problems of prosecuting Putin?
"The difficulty would be getting hold of him and getting hold of his cronies," Mr Jordash said.
"He's unlikely to leave Russia for the foreseeable future.
"It's going to be impossible to arrest him unless there's a complete change in government in Russia.
"I wouldn't expect him to be prosecuted any time soon - but I certainly wouldn't rule it out in the longer time.
"As every dictator should be aware - as Gaddafi should have been aware, as Saddam Hussain should have been aware - political situations move very rapidly in this fast moving world.
"In the long-term, international justice has a very good record of eventually getting those that they target."
If Mr Putin was charged with war crimes and he travelled to a country that operates universal jurisdiction, that nation would be obliged to detain him, Mr Jordash said.
In 1998, former Chilean dictator General Augusto Pinochet was arrested in London on charges of genocide and terrorism - the first time that a former government head was arrested on the principle of universal jurisdiction.
However Gerry Simpson, a professor of public international law at the London School of Economics, warned that prosecuting Mr Putin for war crimes during the Ukraine conflict could be "politically amateurish".
He told Sky News: "I'm not sure war crime investigations at this point in a war are always a particularly good idea.
"This is a tricky area. It may be if Putin remains the president of Russia we'll need to negotiate with him.
"Negotiating with a suspected war criminal - someone who's been charged with war crimes - is rather difficult.
"It could be that war crime prosecutions - a good idea as they seem from a moral or legalistic perspective - can be a bit politically amateurish."
Could other Russians be prosecuted?
Mr Jordash told Sky News that it was possible that "hundreds" of political and military figures from Russia could be tried for war crimes.
He said: "In the international courts, they tend to focus on the higher-ranking people, as a matter of policy and a matter of resources, but that's changing.
"That's changing as a result of recognition that it's much harder these days… to arrest high-ranking members of perpetrating governments.
"There's a movement at the international level to prosecute lower rank people.
"Potentially, there's no reason why hundreds could not be tried from the Russian military and political leadership."
Who decides if a war crime has occurred?
The ICC investigates and tries people charged with genocide, war crimes, crimes against humanity and the crime of aggression.
The court, which sits in the Hague in the Netherlands, began operating in July 2002 and, so far, 45 individuals have been indicted by the court.
They include Ugandan rebel leader Joseph Kony, former President Omar al-Bashir of Sudan, ex-Kenyan President Uhuru Kenyatta and former Libyan head of state Muammar Gaddafi.
What are the punishments for war crimes?
If Mr Putin was convicted of war crimes, he may receive a life sentence and could potentially serve his jail term in the UK, Mr Jordash said.
The lawyer told Sky News: "Any sentence above 20 years is effectively a life sentence for him anyway since he's 70 years old.
"If it was a situation where the ICC prosecuted him and he was found responsible, then he would be detained in one of the states that has come to an agreement with the ICC to take ICC convicted persons - so it could be anywhere in Europe basically. That includes the UK.
"The UK, for example, has detained Radovan Karadzic (the former Bosnian Serb leader convicted of genocide) and Charles Taylor (the ex-Liberian president convicted of war crimes), so the UK does take some of these high-ranking convicted persons."
Prof Simpson said there had been criticism that some sentences handed out by the ICC were "too light" and the prison conditions for those convicted are "comparatively luxurious to those you might expect in a national trial".