Roman Herasymenko, 57, was shot in his hometown in Ukraine by Russian soldiers, his mother said, as he tried to recover a SIM card from his mobile phone which they had smashed. He struggled home but died hours later.

"He walked 50 metres away and they shot him," said 81-year-old Taisia Herasymenko, repeating an account she was given by her son in his final hours in Borodianka, a town in the Bucha district, northwest of Kyiv.

"That's it. I've lost my child," she said, weeping as she clutched his death certificate and demanded his killers be held to account: "They have to try them in court."

Ukrainian and international officials have already launched a range of war crimes investigations, including in the Bucha area where Herasymenko died, in the six months since Russia launched its invasion. But they say bringing perpetrators to justice can take years and cases need to be prioritised.

Herasymenko was shot on March 17, less than a month after Russian tanks rolled into Ukraine. Ukraine's authorities later found mass graves in the Bucha area and what they said was evidence of executions and torture.

Herasymenko's body was among roughly 250 victims recovered in the weeks after Russia withdrew its forces from the area, Borodianka police chief Viacheslav Tsyliuryk said.

His body was exhumed on April 30 and taken to a morgue to determine the cause of death, although further investigation was needed to determine who fired the shot, Tsyliuryk said.

"We can establish an army unit, but at the moment it is not possible to hold a specific individual responsible," he said.

Russia's Defence Ministry did not immediately reply to questions about Herasymenko's shooting. Moscow has repeatedly denied targeting civilians in what it calls its "special military operation" in Ukraine. It also denies accusations its forces have committed war crimes. It has said Ukraine fabricated graves in Bucha.

'NO POSSIBLE JUSTIFICATION'

External experts are in Ukraine to assist the country's war crimes unit in gathering evidence and witness statements, even as fighting rages mainly in the east and south of Ukraine. Their aim is to prepare cases that could meet the rigorous legal threshold required by an international court.

British attorney Nigel Povoas, lead prosecutor with U.S.-backed Atrocity Crimes Advisory Group, has conducted an initial survey of a residential area of Borodianka where apartment blocks were damaged or destroyed, allegedly by Russian warplanes.

"On the face of it, and at this stage it's very early, this would seem to be an obvious war crime for priority investigation," Povoas told Reuters, speaking next to one flattened apartment block.

Based on preliminary findings, Povoas said there appeared to be no military justification for strikes on the residential area. "To intentionally target civilians or civilian infrastructure, or buildings, is a war crime," he said.

Although a probe might take years, he said, "there are effective means to identify commanders in the forces all the way up the chain."

Russia's Defence Ministry did not immediately respond to a Reuters request for comment about the strike on the residential area of Borodianka.

The International Criminal Court in The Hague has launched its own investigations into possible war crimes, crimes against humanity or genocide in the conflict in Ukraine, since the invasion began on Feb. 24.

U.N.-backed tribunals have brought to trial more than 200 people suspected of war crimes in conflicts ranging from Rwanda to Cambodia and Sierra Leone to the former Yugoslavia. Some have been former heads of state. Suspects involved in other conflicts, such as Syria and Yemen, have not been tried.

The Ukrainian authorities say they have opened more than 26,000 cases of possible war crimes since the war began and its judiciary has charged about 150 individuals so far. Moscow has dismissed the accusations.

Russia has also opened criminal cases against Ukrainians for alleged torture of Russian soldiers, charges Kyiv denies.

Yuriy Belousov, Ukraine's lead prosecutor for human rights violations, said the case of Taisia Herasymenko's son could be added to the list of cases being probed.

"Each of these ... has to be investigated. It is a matter of time. But we definitely have to prioritise," he said.

Yet, even as Taisia Herasymenko seeks justice for her son, the conflict has brought more tragedy to the family. Her grandson was killed in fighting in east Ukraine.