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UK Foreign Secretary Liz Truss said on Sunday she was "appalled" at reports Ukrainians captives were being shipped to forced-labour camps in Russia.
With the city of Mariupol under constant siege by Russian troops, there have been claims from Ukraine MPs that Moscow has sent some citizens to "distant parts of Russia" to work without payment.
Ms Truss condemned the "Russian atrocities" in the south-east port city, pledging that President Vladimir Putin would be "held to account".
Elsewhere, UK Prime Minister Boris Johnson, in a phone call with Ukrainian President Volodymyr Zelenskyy on Sunday afternoon, said the UK was "committed to stepping up military, economic and diplomatic support" in a bid to bring about an end to the war.
The reports that thousands of Mariupol's residents have been abducted remains unverified, but Ukrainian MP Inna Sovsun said that, according to information being shared by the city's authorities, her countrymen and women were effectively being forced into slave labour.
"The very logic of Russia right now is the logic of the Soviet Union and the logic of Nazi Germany," Ms Sovsun told Times Radio.
"It is a completely totalitarian state, and it is acting as one. So from what we know from the city mayor and the city council, they are taking Ukrainian citizens.
"They are sending them through what are called the 'filtration camps' and then they are being relocated to very distant parts of Russia, where they are being forced to sign papers [saying] that they will stay in that area for two or three years and they will work for free in those areas."