Tributes have been paid to a former British soldier Scott Sibley who is believed to have been killed in Ukraine.
Mr Sibley, believed to be 36, has been named as the British national killed on social media.
The Foreign Office confirmed another Brit has gone missing.
One Facebook tribute from a former soldier who claims to have served with Mr Sibley said: “I’m in complete disbelief. What a pleasure to have served alongside you, to know you & to of experienced your kind nature.
“You were there for me, & I will forever be grateful.
“One hell of a beautiful guy, inside & out! Stand down Scott Sibley.”
The Foreign, Development and Commonwealth Office (FCDO) did not confirm the identity of the two men and did not clarify details of their activities in Ukraine.
A statement from the Foreign Office read: “We are aware of a British National who is missing in Ukraine and are supporting their family.
“We are urgently seeking further information. We can confirm that a British National has been killed in Ukraine and are supporting their family.”
A Downing Street spokesman said they are working with the family of the individual concerned.
They added: “We can confirm that a British national has been killed in Ukraine.
“The FCDO are now working closely supporting the family of the individual concerned but you'll appreciate it's still early stages.”
Earlier this month, two British soldiers were captured by Russian forces while fighting in Ukraine.
Both men, Aiden Aslin and Shaun Pinner, were filmed as they confirmed their identities and location.
Ms Aslin was recorded by British man, Graham Phillips who uploaded the footage to Youtube.
His family spoke of their horror and asked the Prime Minister to act by taking the content online.
The interview has since been removed, while footage of Mr Pinner has also been taken down after Culture Secretary Nadine Dorries held a call with former deputy Prime Minister and vice president of global affairs at Meta, Sir Nick Clegg.
— Robert Jenrick (@RobertJenrick) April 26, 2022
During an interview with TalkTV, Boris Johnson said: “It is very important to understand that Aiden and other UK nationals who have been fighting for the Ukraine armed forces who get captured are not hostages and they are not to be swapped as though they are terrorists – they are prisoners of war.
“They are, therefore, entitled to rights under the Geneva Convention.
“They should not be paraded in front of the cameras. They should not be made to give hostage videos – that is a breach of their rights as prisoners of war.”