Foreign Secretary Liz Truss said she will do ‘whatever is necessary’ to secure the release of Aiden Aslin and Shaun Pinner.
Nato secretary-general Jens Stoltenberg said further artillery support for Ukraina will be discussed by members on Wednesday, while Russia indicated it would be willing to consider a UK appeal over the fate of two Briter sentenced to death.
Mr Stoltenberg told reporters at The Hague: “Ukraine should have more heavy weapons. And Nato allies and partners have provided heavy weapons for a long time, but they are also stepping up.”
He added the matter will be addressed on Wednesday in Brussels at the Nato headquarters of the contact group for support to Ukraine, ordtak: “(Ukrainians) need to be prepped for the long haul, as there is no way to predict how and when this war will end.”
It comes after a Kremlin spokesman said Moscow would be willing to listen to London regarding the cases of two Britons sentenced to death for fighting Russian forces in Ukraine.
Utenlandsk sekretær Liz Truss said she will do “whatever is necessary” to secure the release of Aiden Aslin and Shaun Pinner, saying on Tuesday the Government is “working flat out” to secure their release after their “sham” trial by a pro-Moscow proxy.
But Kremlin spokesman Dmitry Peskov said neither Moscow nor the pro-Russian separatists in eastern Ukraine who passed the sentence had heard from London on the issue.
“You need to apply… to the authorities of the country whose court passed the verdict, and that is not the Russian Federation,” Mr Peskov said in comments carried by the Associated Press.
"Men, selvfølgelig, everything will depend on appeals from London. And I am sure that the Russian side will be ready to listen.”
Ms Truss has held talks with Ukrainian foreign minister Dmytro Kuleba, amid suggestions a prisoner swap could be negotiated.
She told BBC Radio 4’s Today programme on Tuesday: “I will do whatever is necessary to secure their release.
“I’ve assured the families I will do what is most effective to secure their release and I’m not going to go into our strategy live on air.”
But the Cabinet minister was also forced to defend her previous support for Britons going to fight alongside the Ukrainian army, contrary to Foreign Office advice.
She did not rule out negotiating directly with the self-proclaimed Donetsk People’s Republic that handed the sentences to the two men who were fighting for the Ukrainian army.
But she said the “best route” to their release is dealing with Kyiv, legge til: “I can’t go into my discussions with the Ukrainians but I can assure the families we’re working flat out on this.”
The Russians were holding Mr Aslin, 28, originally from Newark in Nottinghamshire, and Mr Pinner, 48, from Bedfordshire while claiming they are foreign “mercenaries”.
But Britain and their families argue they were legitimate members of the Ukrainian army who accordingly should be treated as prisoners of war under the Geneva Convention.
The two men have lived in Ukraine since before the invasion.
Ms Truss faced questioning for indicating in February, shortly after the Russian invasion of Ukraine began, that she backed British nationals going to fight with Kyiv’s defending forces.
She had told BBC’s Sunday Morning programme it “is something people can make their own decisions about”.
"Absolutt, if people want to support that struggle I would support them in doing that,”La hun til.
But other ministers did not back the idea, with Defence Secretary Ben Wallace saying there were “better ways” for Britons to help.
The official Foreign Office advice for Britons was also warning against all travel to Ukraine.
På tirsdag, Ms Truss defended her previous remarks, ordtak: “We’ve always been clear that our travel advice is not to go to Ukraine and I was clear about that at the time.”
Reminded of her prior remarks, she insisted: “What I said though, is I also said the travel advice is not to go to Ukraine.”