Prime Minister Andy Burnham has said he will try to "repair bridges" between Ukraine and the United States, ahead of a meeting with President Volodymyr Zelensky at Downing Street.
The Ukrainian president was welcomed by a military ceremonial guard outside Number 10 as he arrived for the meeting, in what is thought to be a first for a foreign leader.
It comes after President Donald Trump said he had struck a deal with Moscow to import Russian diesel.
Asked about Trump's move during Prime Minister's Questions, Burnham resisted criticising the US president, saying he wanted to "be careful in terms of laying the ground to bring people back together".
However, he stressed that Zelensky would not be leaving the UK "empty-handed", as he said further military and financial support would be drawn up in the coming weeks.
Burnham is expected to use the visit to promise further sanctions against Russia, while also announcing the UK will accelerate its delivery of air defence missiles to Kyiv.
The UK will also urge allies to step up pressure on Russia, with Chancellor John Healey using International Monetary Fund meetings to call for a ban on Russian liquefied natural gas (LNG).
It is Zelensky's second visit to the UK since Burnham became prime minister.
While other world leaders have received military welcomes, Number 10 said it could not find a record of one taking place on Downing Street.
The King held an audience with President Zelensky at Buckingham Palace as part of his visit to the UK
During PMQs, Liberal Democrat leader Sir Ed Davey asked whether he agreed the diesel deal was "yet another shameful Trump betrayal of the Ukrainian people and European security".
In response, Burnham did not echo Sir Ed's condemnation of Trump, saying: "What I have to do in my role is try and repair bridges so that the support comes back and therefore I should at this moment in time be careful in terms of laying the ground to bring people back together."
The PM said he had spoken to Zelensky over the phone on Saturday "because I recognised it was another very dark and difficult moment for him as they contemplate a winter that will be severe for his people".
The phone call came after Trump announced that President Vladimir Putin had agreed to release millions of tonnes of Russian diesel into the US, as he suspended sanctions on diesel from Moscow.
Zelensky branded the move a gift to Putin, warning it would provide Moscow with money to continue its war against Ukraine.
Trump responded by saying it was "time for Ukraine to get a new president".
Meanwhile, Zelensky has said at least 600,000 people across Ukraine were left without electricity after deadly overnight Russian missile and drone strikes.
He said "critical" energy infrastructure had been hit – despite Trump's claim on Sunday that a ceasefire on strikes on energy infrastructure was in place between Russia and Ukraine.
Russia said its strikes had hit communications and energy facilities used by Ukraine's military.
Zelensky has said Ukraine is ready to halt its strikes on Russian oil refineries and other energy facilities if Moscow did the same.
But Russia's Foreign Minister Sergey Lavrov has said Ukraine does not deserve the "respite" a ceasefire would bring.
Analysis: Burnham careful ahead of meeting with Zelensky
Why Moscow is staying tight-lipped on Trump's energy ceasefire
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