Ukraine President Volodymyr Zelensky on Monday visited the town of Bucha outside the capital Kyiv where dozens of corpses, some with their hands bound, were discovered after Russian forces withdrew.

"Every day, when our fighters enter and retake territory, you see what's been happening," the Ukrainian leader told reporters in the town devastated by fighting, wearing a bullet-proof vest and accompanied by military personnel.

"These are war crimes and it will be recognised by the world as genocide," Zelensky.

"You stand here today and see what happened. We know that thousands of people have been killed and tortured with extremities cut off, women raped, children killed," he told reporters wearing a bullet-proof vest and accompanied by military personnel.

"It's genocide," he added.

Ukraine and Western nations have accused Russian troops of war crimes after the discovery of mass graves and apparently executed civilians at Bucha, prompting vows of action at the International Criminal Court.

Russia's defence ministry denied its troops had killed civilians in the town recently retaken by Ukrainian soldiers.

Britain, France, Germany, the United States and NATO all voiced horror at the images from Bucha, where some of the bodies lying in public appeared to have been bound by their hands and feet before being shot.