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Scott Sibley, a British national, has been killed in Ukraine and a second person has gone missing.
Sky News reported that the pair may have been volunteers fighting against Russian forces, but this has not been confirmed.
"We can confirm that a British national has been killed in Ukraine and are supporting their family," the UK Foreign Office said in a statement, without giving further details.
The statement also addressed the missing person.
"We are aware of a British national who is missing in Ukraine and are supporting their family. We are urgently seeking further information."
The UK government advised people not to go and fight in Ukraine but to support the country in different ways.
The advice has been ignored by several British citizens, among them Aiden Aslin, who was captured by the Russian army more than a week ago.
Mr Aslin's family begged Russian President Vladimir Putin to “treat him with humanity” after images on Russian television appeared to show him along with fellow British prisoner Shaun Pinner being led around in handcuffs with a cut on his forehead.