A veteran of the British soldier Scott Sibley has died and a second British man has gone missing after they are believed to have travelled to Ukraine to fight Russian troops.
Scott Sibley, 36, is the first British citizen to die in the war.
A friend described him as the bravest man he knew. Another said he had a "bad sense of humour."
It is also unknown on what day he was killed, but it is believed that it was a few days ago in the south or east of Ukraine.
Why they ended up in Ukraine has not been confirmed. A spokesman for the International Legion, which includes foreign fighters in the country, declined to comment.
Sky News decided not to name the missing person. The families of both British citizens were informed about the situation and received support from the Ministry of Foreign Affairs.
The Office for Foreign Affairs, Commonwealth and Development (FCDO) did not provide any details about what men were doing in Ukraine.
An FCDO spokesman said: "We can confirm that a British citizen was killed in Ukraine and is supporting his family."
Mr. Sibley previously served in the Royal Logistics Corps.
The Ministry of Foreign Affairs, Development and Commonwealth Affairs (FCDO) did not confirm the identities of the two men and did not specify the details of their activities in Ukraine.
A statement from the Foreign Office said: "We know of a British citizen who went missing in Ukraine and is supporting their family.
"We are urgently looking for additional information. We can confirm that a British citizen was killed in Ukraine and he is supporting his family. "
A spokesman for Downing Street said they were working with the family of the person concerned.
They added: "We can confirm that a British citizen was killed in Ukraine.
"FCDO is now working closely together to support the person's family, but you will understand that this is still at an early stage."
Earlier this month, two British soldiers were taken prisoner by Russian troops during a battle in Ukraine.
Both men, Aiden Aslin and Sean Pinner, were videotaped when they confirmed their identities and locations.
Aslin was recorded by British Graham Phillips, who uploaded the footage to Youtube.
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