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Images of cluster bombs and artillery strikes on Ukrainian cities this week have prompted the world's top war crimes prosecutor to launch an investigation, with the support of dozens of nations opposed to Russia's invasion.
Karim Khan, a British lawyer named as the chief prosecutor of the International Criminal Court last year, said the crisis in Ukraine is a chance to demonstrate that those committing war crimes would be held to account.
"I think the world is watching. The world expects better," Khan, 51, said on Thursday in his offices in The Hague, announcing he had dispatched an advance team of investigators and lawyers to the region.
"Any side that targets - directly targets - civilians or civilian objects is committing a crime."
According to the United Nations, at least 249 civilians have been killed since Russian tanks rolled into Ukraine on Feb. 24, in the biggest military attack on a European state since World War Two.
Ukrainian emergency services have placed the civilian death toll nearly 10 times higher.
Like all U.N. states, Russia and Ukraine are subject to the 1949 Geneva Conventions - which established legal standards for humanitarian treatment in war and outlawed deliberate attacks on civilians.
Ukrainian President Volodymyr Zelenskiy has accused Russian troops of intensifying their bombardment of civilians in recent days, as the Russian forces laid seige to cities. An unprecedented 39 member states petitioned the ICC this week to open an investigation, including most European Union states and Britain, which publicly accused Russian President Vladimir Putin's government of war crimes.
Kremlin spokesperson Dmitry Peskov strongly denied this.
"Russian forces are not launching any strikes at civilian infrastructure targets or residential complexes,” Peskov told Reuters on Tuesday.
Moscow says it is conducting a "special operation" to demilitarize Ukraine and has in turn accused Kyiv of genocide against Russian speakers. Kyiv strongly denies this.
Past cases at the ICC and U.N. war crimes tribunals suggest that - despite the mounting civilian casualties - Khan's team faces a long and difficult process to prove violations of international humanitarian law in Ukraine and to prosecute those responsible.
Noting that his office is overstretched as it conducts 16 cases and trials worldwide, Khan himself acknowledged that the Ukraine investigation represents "myriad challenges, opportunities and difficulties".
This week's request by member states enabled the ICC to open a fast-track inquiry on Ukraine after eight years of preliminary work on events dating back to Russia's annexation of the Crimean peninsula in 2014, but Khan said his office is scrambling for resources.
"We need everything," he said. "From lawyers to investigators, to analysts and military experts, to forensics, language experts and psycho-social support."
A successful prosecution requires meticulously documenting incidents and preserving evidence in a war zone. But, as the ICC does not command its own police force, it must rely on national governments for intelligence, the provision of evidence, and the detention of any suspects for transfer to The Hague.
Although neither Russia nor Ukraine are among the ICC's 123 member countries, Ukraine gave prosecutors authorisation to investigate crimes under the court's jurisdiction -- meaning that Russian or Ukrainian soldiers, military commanders and politicians could in theory be tried before the ICC for any war crimes committed.
To qualify as a war crime, an act must be proven to have been deliberate or indiscriminate, said Juliette McIntyre, a law lecturer at the University of South Australia and a specialist in international courts and tribunals.
"There is obviously a lot of emotion involved when we see buildings being blown up, but international humanitarian law - or the law of war - doesn't absolutely prohibit harm to civilians or their property," McIntyre said. "It tolerates some collateral damage."
PUTIN A TARGET?
Astrid Reisinger Coracini, lecturer at the University of Vienna Department of International Law, said one of the major challenges the ICC team faces is finding evidence to link crimes on the ground to leaders higher in hierarchy who gave orders.
"And the higher it goes, the more difficult it becomes," she said.
Since it opened its doors in 2002 to prosecute the gravest atrocities, the ICC has secured five convictions for war crimes, crimes against humanity and genocide from 38 defendants.
Several cases are ongoing; four suspects were acquitted, three died and 10 remain at large, including Sudan's former President Omar al-Bashir.
However, international tribunals have convicted numerous political leaders for war crimes, including Liberia's former president Charles Taylor.
Asked if the ICC could pursue senior politicians including Putin himself for any crimes committed in Ukraine, Khan did not directly respond but cited the Taylor precedent: "It's very clear ... if crimes are committed - if there is evidence to support it - the writ of law will allow prosecution."
British attorney Wayne Jordash, who represented high-level suspects at the U.N. tribunal for the former Yugoslavia, said ICC prosecutors should focus on Putin after his public comments and a recent live television appearance with his security team show "he's fully in control".
Jordash added that Putin had a long history of intervention in Ukraine, dating back to the annexation of Crimea and support for rebel groups in the eastern region of Donbass.
"We have cluster bombs (in the current conflict). We have hitting of civilian targets, which cannot and do not look as though they are accidents," said Jordash, who has helped collect evidence of atrocities in Ukraine since 2014. "Even if there wasn't sufficient (evidence) to move to an investigation before, there certainly is now."
Russia has consistently denied backing the pro-Moscow rebels in Ukraine and says that Crimea voted to join Russia in a disputed 2014 referendum.
The use of cluster munitions - which experts told Reuters were fired on Ukraine's second-largest city of Kharkiv on Monday - is not prohibited in itself because neither Russia nor Ukraine has joined a 2008 convention banning them.
However, it could be a war crime if used in an indiscriminate attack or one intentionally targeting civilians, six experts interviewed by Reuters said, referring to the Rome Statute that established the ICC and defines the nature of the crimes it may try.
SYRIA, CHECHNYA
Three experts interviewed by Reuters cited Russia's track record of conducting campaigns in civilian areas, including air strikes that hit hospitals in Aleppo, Syria, in 2016, and the 1999-2000 battle of Grozny during the Second Chechen War, which left the city destroyed.
Russia has denied indiscriminate attacks on civilian targets in Syria, Chechnya and now Ukraine.
According to Human Rights Watch, among the most serious incidents in the last week that may violate the rules of war was a Russian ballistic missile carrying a cluster bomb that struck outside a hospital in the eastern town of Vuhledar on the first day of the invasion, killing four civilians and injuring 10.
Amnesty International also reported an explosive weapon, which it described as likely an artillery shell, that hit a kindergarten in Chernihiv on Feb. 26.
Intentional attacks on schools and hospital facilities are expressly cited as war crimes under the Rome Statute, as are assaults on purely civilian areas with no military defences.
In Kharkiv, officials told Reuters that dozens of civilians had died in Russian artillery barrages on residential areas this week.
"The Russian enemy is firing at entire residential areas of Kharkiv, where there is no critical infrastructure, where there are no Ukrainian army positions that the Russians could be targeting," town official Oleg Synegubov said.
Reuters was unable to independently verify the three incidents.
Dr Carrie McDougall, senior lecturer at the University of Melbourne, said that even if prosecutors pursue Putin, 69, there was little chance he would stand trial in The Hague while he remains in office - which he could do until 2036 following a law introduced last year.
But McDougall envisaged the scenario would shift if Putin loses power, given there is no statute of limitations on war crimes and no immunity possible.
"So Putin and the officials around him could be held responsible for any war crimes that are committed," she said. "Whether it's in 5 years or 10 years or 15 years ... we're talking about the long arm of justice."
(Reporting by Anthony Deutsch and Toby Sterling in Amsterdam; Additional reporting by Natalia Zinets in Lviv, Stephanie Nebehay in Geneva and Stephanie van den Berg in The Hague; Editing by William Maclean and Daniel Flynn)