A leading rights group said on Sunday it had documented what it described as "apparent war crimes" committed by Russian military forces against civilians in Ukraine.
Human Rights Watch (HRW) issued a statement saying it had found "several cases of Russian military forces committing laws-of-war violations" in Russian-controlled regions such as Chernihiv, Kharkiv, and Kyiv.
The statement, published in Warsaw, came one day after dead civilians were found lying scattered through the streets of the Ukrainian country town of Bucha, three days after the Russian army pulled back after a month-long occupation of the area 30 km (20 miles) northwest of Kyiv.
The Russian defence ministry in Moscow did not immediately reply to a request for comment when asked on Sunday about the bodies found in Bucha and the HRW statement.
The Kremlin says its "special military operation" aims to degrade the Ukrainian armed forces and is targeting military installations and not carrying out strikes on civilians.
Asked about separate war crime allegations on 1 March, Kremlin spokesman Dmitry Peskov told a conference call with reporters, "We categorically deny this". He dismissed allegations of Russian strikes on civilian targets and the use of cluster bombs and vacuum bombs as fakes.
Defence Ministry spokesman Igor Konashenkov said on 21 March that Russia's operation was being carried out by a professional and well-armed forces and denied Ukrainian claims that Russian forces had hit any civilian objects.
The New York-based HRW referred to Bucha in its statement, for which it said it had interviewed 10 people, including witnesses, victims, and local residents, in person or by telephone and that some were to scared to give their full names.
"The cases we documented amount to unspeakable, deliberate cruelty and violence against Ukrainian civilians," said Hugh Williamson, HRW's Europe and Central Asia director.
"Rape, murder, and other violent acts against people in the Russian forces' custody should be investigated as war crimes."
These, it said, included a case of repeated rape; two cases of summary execution - one of six men - and other cases of unlawful violence and threats against civilians between 27 February and 14 March, 2022.
"Soldiers were also implicated in looting civilian property, including food, clothing, and firewood. Those who carried out these abuses are responsible for war crimes," the report said.
Reuters was not immediately able to verify the HRW evidence.
Reuters journalists visited Bucha on Saturday, after being given access by Ukrainian forces who recaptured the area, and saw bodies wearing no military uniforms scattered in the streets.
HRW said on 4 March Russian forces in Bucha, "rounded up five men and summarily executed one of them."
Northeast of Kyiv in the Chernihiv region, the report said, Russian forces in Staryi Bykiv rounded up at least six men on 27 February, later executing them. It cited the mother of one of the men, who said she was nearby when her son was captured and who later saw the bodies of all six men.
HRW said all parties to the armed conflict in Ukraine were obligated to abide by international law and the laws of war.
"Russia has an international legal obligation to impartially investigate alleged war crimes by its soldiers," Williamson said.