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Two British aid workers are believed to have been captured by Russian forces in Ukraine, according to a humanitarian group that had been in contact with them.
The non-profit Presidium Network said the two volunteers, whom it named as Paul Urey and Dylan Healey, were last heard from on Monday.
It said a family they were helping in southern Ukraine had since been interrogated by Russian forces, who claimed that the two British men were spies.
Dominik Byrne, a co-founder of Presidium, said it was very likely that the aid workers had been captured after trying to help the family out of the city of Zaporizhzhia, the site of Russian attacks during the two-month invasion.
The UK government said it was urgently seeking more information about the two people. The news came a day after it was announced a British citizen, Scott Sibley, was killed in Ukraine and a second person had been reported missing.
Russia separately released a video on Friday showing a bandaged man identified as Andrew Hill, who spoke with a British accent, purporting to identify himself as a foreign mercenary. British soldiers Shaun Pinner and Aiden Aslin were earlier paraded on Russian state television.
Cabinet minister Anne-Marie Trevelyan, speaking for the government on morning television, said the UK was working with local authorities to help British citizens in Ukraine but that people were advised against travelling to the country.
Mr Byrne told BBC Breakfast that the two volunteers were not working directly for Presidium but that he had been trying to bring them into the fold because he was worried for their safety.
He said the group had seen text messages indicating that the volunteers were helping a woman and her two children to escape from Zaporizhzhia into territory farther south, which is occupied by Russia.
Their correspondents lost contact with them on Monday morning, and that afternoon the woman who had escaped Zaporizhzhia received "strange text messages" that purported to be from one of the volunteers, but were not consistent with their previous texts.
"She got a text message saying that they're five minutes away," Mr Byrne said. "Two hours later, her house was stormed by Russian soldiers."
According to the woman's account, the Russian troops shouted at her and asked "why do you know those two British people? You shouldn't be talking to them, we think they're spies".
Presidium describes itself as a non-profit group that has previously offered help to people escaping Afghanistan and is now providing medical supplies and evacuation assistance in Ukraine.
The apparent capture comes amid escalating rhetoric between the UK and Russia. Almost 300 British MPs were this week sanctioned by Moscow, which previously banned Prime Minister Boris Johnson and other top officials from entering the country. Britain had previously sanctioned top Russian figures.
Russia also accused Britain of provoking attacks on its territory after Armed Forces Minister James Heappey said it was legitimate for Ukraine to strike Russian targets in self-defence.
The Russian Defence Ministry responded to that by threatening a "proportional response" if such attacks took place. Western officials said any such retaliation would be unlawful.