The Foreign Office is investigating reports that a British national has been detained by Russian forces after a video emerged showing a man in military clothing being questioned.
The footage, reportedly broadcast on Russian television, shows the man appearing to give his name as Andrew Hill. He speaks with an English accent, has his arm in a sling, a bandage around his head, and blood can be seen on his hand.
The video, which has not been verified, has also been shared online.
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Earlier on Saturday, Conservative MP Tobias Ellwood – a former captain in the British Army – warned against veterans going to Ukraine to help fight in the war.
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The Foreign, Commonwealth & Development Office (FCDO) is investigating the reports and supporting family members.
Two other British men said to be working as humanitarian aid volunteers are also believed to have been detained in Ukraine by Russian forces.
The Presidium Network, a non-profit group, said Paul Urey and Dylan Healey were captured early on Monday morning at a checkpoint south of the city of Zaporizhzhia in south-eastern Ukraine.
Mr Urey, from Manchester, and Mr Healey, from Cambridgeshire, travelled to Ukraine of their own accord, the organisation said.
They were not working for the Presidium Network, which helps get aid into Kyiv.
The organisation said the pair were driving to help a woman and two children evacuate when they went missing.
Presidium Network said it is concerned that Russian forces may think the two men are British spies.
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Meanwhile, in an intelligence update on Saturday, the Ministry of Defence (MOD) said Russia faces "considerable challenges" and troops are likely to be suffering from "weakened morale".
Posting on Twitter, the MOD said: "Russia hopes to rectify issues that have previously constrained its invasion by geographically concentrating combat power, shortening supply lines and simplifying command and control.
"Russia still faces considerable challenges. It has been forced to merge and redeploy depleted and disparate units from the failed advances in north-east Ukraine.
"Many of these units are likely suffering from weakened morale.
"Shortcomings in Russian tactical co-ordination remain. A lack of unit-level skills and inconsistent air support have left Russia unable to fully leverage its combat mass, despite localised improvements."
On Saturday, President Volodymyr Zelensky tweeted: "I keep in touch with @BorisJohnson. Spoke about the situation on the battlefield and in the blocked Mariupol.
"Discussed defensive support for Ukraine and the necessary diplomatic efforts to achieve peace."