At least 8,000 people were killed by fighting or war-related causes in Russia's months-long conquest of Mariupol, one of the biggest battles of the nearly two-year war between Russia and Ukraine, according to Human Rights Watch.

Mariupol became a byword for horror during a nearly three-month-long Russian siege for control of the strategic port city between March and May 2022, with trapped civilians forced to bury their dead by the roadside.

The Human Rights Watch assessment, based on satellite and other images of grave sites, is one of the only independent estimates of the death count so far.

Ukraine has said tens of thousands were killed but that it cannot give an exact number without access to the city, now under Russian control.

The United Nations, which has rights monitors on the ground in Ukraine, has so far documented more than 10,000 civilian deaths in the country overall since Russia's invasion but says it has not yet been able to corroborate reports of high civilian deaths in Mariupol due to access constraints.

Russia has vigorously denied committing atrocities or targeting civilians in Ukraine.

Human Rights Watch said the total could be significantly higher than its estimate since some graves have been known to contain multiple bodies and some sites might not have been identified.

The full 224-page report 'Our City Was Gone: Russia's Devastation of Mariupol, Ukraine' compiled with NGO Truth Hounds and architecture practice SITU, also draws on some 240 interviews with mostly displaced Mariupol residents.

HRW documents attacks that struck civilian infrastructure, including hospitals, a supermarket and a theatre sheltering civilians. It found no evidence of a Ukrainian military presence at these sites or nearby, which it said made the attacks unlawful.

"Russian forces' devastation of Mariupol stands out as one of the worst chapters of their full-scale invasion of Ukraine," said Ida Sawyer, crisis and conflict director at Human Rights Watch, calling for governments to investigate.

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The report names 10 people with "command responsibility" it said should be the focus of possible war crimes investigations.

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