Ukrainian investigators began to exhume a mass grave in Bucha on Friday, opening the early stages of what police say will be a war crimes case targeting Russian troops who occupied the Kyiv commuter town.

The grave -- a long deep trench in the mud behind a gold-domed church -- was used by Ukrainians to bury neighbours they claim were killed at the hands of Russian armed forces which arrived on February 26.

On Friday, crews wearing white forensic overalls were using a flatbed lorry fixed with a mechanical crane to remove corpses from the ditch.

The bodies were laid out on the ground and inspected by teams, including police officers poring over paperwork.

Most of the remains outside the grave, cordoned off by tape, were zipped inside black plastic body bags.

One man at work was wearing a gilet marked with the words "war crimes prosecutor".

Kyiv regional police chief Andriy Niebitov said there were 40 bodies in the grave, including two members of Ukraine's military forces.

An aerial shot of Bucha, showing a mass grave near St Andrew church
An aerial shot of Bucha, showing a mass grave near St Andrew church AFP / RONALDO SCHEMIDT

He said bodies bore gunshot wounds, buttressing claims they were explicitly targeted by soldiers rather than collateral damage from air strikes and artillery fire.

"I can define these events as a war crime," he said. "International law defines killing of civilians during any kind of military conflict as a war crime."

"These bodies will be extracted for the court investigation, they will be delivered to undergo forensic medical examination and an autopsy."

The town of Bucha has become the growing focus of allegations of war criminality against troops ordered to invade Ukraine by President Vladimir Putin on 24 February.

On Saturday an AFP team discovered 20 dead bodies on a single street in the town, home to around 37,000 before the war.

Ukraine says Russian troops executed civilians.

The Kremlin has denied any hand in the killings and denounced photographs depicting the scene as fakery.