The bodies of 200 people have been discovered inside a basement in Mariupol, local authorities have said.
Workers digging through the rubble of an apartment building made the grim discovery, with the number of victims found making it one of the deadliest attacks of the war.
An adviser to the region’s mayor, Petro Adryushchenko said the bodies were decomposing and a “stench” covered the surrounding area.
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“The city has turned into a continuous cemetery,” he said in an online post, adding that a “makeshift morgue” had been set up to deal with “a large number of corpses”.
It is the latest horror to come to light in the key port city, which has been an important target for Russian forces since their invasion started three months ago.
Last week, Ukrainian troops were told to stop defending the southern Ukrainian city and to “save the lives of soldiers” instead.
Stationed at the Azovstal steel plant, which quickly became a focal point in the war, the 2,500 soldiers were considered the be the last ones fighting Russian forces in the area.
Russian troops already held the rest of the city, where an estimated 100,000 people remain, many of them trapped during the siege with little food, water, heating or electricity.
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Ukrainian authorities have said at least 21,000 people were killed during the battle for Mariupol, with attacks carried out on places such as a maternity hospital and a theatre.
The country has accused Russia of trying to cover up the horrors of its attacks by bringing in mobile cremation equipment and burying the dead in mass graves.
Ukraine’s President Volodymyr Zelenskyy has accused Russian forces of waging “total war” and seeking to inflict as much death and destruction as possible.
“Indeed, there has not been such a war on the European continent for 77 years,” Mr Zelenskyy said, referring to the end of the Second World War.
Russia has ‘increased the intensity’ of operations
Meanwhile, Russia has made some “localised successes” in its bid to take control of the eastern Donbas region, with Severodonetsk and neighbouring cities the only part of the Luhansk region still under Ukrainian control.
The UK’s Ministry of Defence said Russia has “increased the intensity of its operations” in the area but there has been “strong Ukrainian resistance”.
“Russia’s capture of the Severodonetsk pocket would see the whole of Luhansk Oblast placed under Russian occupation,” it added.
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Two top Russian officials appeared to acknowledge that Moscow’s advance has been slower than expected, though they vowed the offensive would achieve its goals.
Nikolai Patrushev, secretary of Russia’s Security Council, said the Russian government “is not chasing deadlines”.
Defence minister Sergei Shoigu said Moscow is deliberately slowing down its offensive to allow residents of encircled cities to evacuate – though forces have repeatedly hit civilian targets.