Amnesty International says it has documented war crimes by Russian forces around Ukraine's capital city.
The organization reported that it had collected evidence and testimony in eight cities near Kyiv, including Bucha.
Images from the city sparked international outrage – though Russia did not take responsibility, calling photos of bodies in the streets staged.
Russia has claimed that it hits only targets with military values, denying allegations of war crimes.
Kyiv regional governor Oleksandr Pavlyuk said that at least 1,235 civilian bodies have been found in the region.
"The pattern of crimes committed by Russian forces that we have documented includes both unlawful attacks and willful killings of civilians," Agnes Callamard, Amnesty International’s Secretary General, said in a Friday statement. "It is vital that all those responsible, including up the chain of command, are brought to justice."
The group's report said the executions in Bucha were carried out with specialized rifles used by some elite Russian units and Amnesty International staff found 7N12 armor-piercing rounds with a 9x39mm black tip at the murder scene, which are used by elite units of the Russian army.
It also says researchers found evidence documenting specific units of the Russian army that were involved.
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"We have met families whose loved ones were killed in horrific attacks, and whose lives have changed forever because of the Russian invasion," Callamard added. "We support their demands for justice, and call on the Ukrainian authorities, the International Criminal Court and others to ensure evidence is preserved that could support future war crime prosecutions."
Reuters reported in April that Ukraine is preparing war crimes charges against at least seven members of the Russian military and is investigating some 7,600 potential war crimes and at least 500 suspects.
The United Nations (U.N.) has also opened its own inquiry into possible war crimes.
The Office of the U.N. High Commissioner for Human Rights (OHCHR) recorded 6,731 civilian casualties, including 3,280 killed. However, OHCHR believes the actual figures are considerably higher.
The U.N. said that, as of Thursday, there were more than 5.7 million refugees from the conflict. The U.N. Office for the Coordination of Humanitarian Affairs (OCHA) said some 7.7 million, or about 17.5% of the total population, are internally displaced.
"This conflict has been causing extreme human suffering, with thousands of civilians killed and injured, and countless others living through daily bombardment and violence," U.N. human rights experts and the Global Protection Cluster Coordinator said in a Friday statement. "Homes, schools, hospitals, care institutions and entire cities have been destroyed. Mines and explosive remnants of war continue to pose alarming threats to civilians, including those remaining in their homes and those fleeing the conflict. We are appalled by the disturbing reports of violations of international human rights law and violations of international humanitarian law which may amount to war crimes."
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The Associated Press contributed to this report.