Ukrainian President Volodymyr Zelenskiy said on Monday (local time) it had become harder for his country to negotiate with Russia since Kyiv became aware of the scale of atrocities carried out by Russian troops in Ukraine.

Zelenskiy spoke on national television from the town of Bucha in the Kyiv region, where tied bodies shot at close range, a mass grave and other signs of executions have been found in territory retaken from Russian troops.

Moscow has denied any accusations related to the killing of civilians in Bucha. 

"These are war crimes and will be recognised by the world as genocide," Zelenskiy said, wearing body armour and surrounded by military personnel.

"It's very difficult to talk when you see what they've done here," he said. "The longer the Russian Federation drags out the negotiating process, the worse it is for them and for this situation and for this war."

"We know of thousands of people killed and tortured, with severed limbs, raped women and murdered children," he said.

After Zelenskiy spoke, Ukrainian officials took journalists to the basement of what they said was a summer residence for children and showed them the bodies of five men with their hands tied behind their backs.

The officials said the five people, who all wore civilian clothes, had been killed by occupying Russian soldiers before Ukrainian troops retook control of the town.

"They were shot, shot either in their head or in their chest. They were tortured before they were killed," said Anton Herashchenko, an adviser to the Ukrainian interior ministry.

"Now we are investigating this and we show (it) to the international press."

He said Russian soldiers had set up camp inside the building and stayed there for three weeks.

Reuters could not independently verify his account.

Bucha's deputy mayor said on Sunday that 50 residents had been victims of extra-judicial killings carried out by Russian troops. 

In nearby Stoianka, about 15 km south of Bucha, Zelenskiy surveyed the damage to a motorway bridge which was almost split in two. One car stood abandoned, bulletholes peppering its bodywork and windscreen. A nearby housing estate was flattened and advertising hoardings were scarred with bulletholes.

Russia denies targeting civilians since invading Ukraine on Feb. 24 in what it calls a "special military operation" aimed at demilitarising and "denazifying" Ukraine. Ukraine says it was invaded without provocation.

The Kremlin on Monday denied any accusations related to the murder of civilians in Bucha.

"This information must be seriously questioned," Kremlin spokesperson Dmitry Peskov told reporters. "From what we have seen, our experts have identified signs of video falsification and other fakes."

The destruction and civilian deaths in Bucha look set to galvanise the United States and Europe into additional sanctions against Moscow, with officials raising the prospect of restrictions on Russia's energy exports.a