British Prime Minister Boris Johnson branded Russia's aggression towards Ukraine "nauseating" during a conversation with the UN secretary-general before he travels to Moscow.
The Prime Minister, in his phone call with Antonio Guterres on Sunday afternoon, "made clear the importance of global solidarity with Ukraine" in the face of "blatant aggression" by the Kremlin, No 10 said.
The United Nations chief has faced criticism from Ukrainian president Volodymyr Zelensky for choosing to visit Russia before he travels to Kyiv. Mr Guterres is scheduled to travel to Turkey on Monday and then Moscow and Kyiv, but Mr Zelensky said it was a mistake for the Portuguese diplomat to arrange his trip in that order. "Why? To hand over signals from Russia? What should we look for?" Zelensky said on Saturday.
"There are no corpses scattered on the Kutuzovsky Prospect," he said, referring to one of Moscow's main avenues.
Offering a readout of Mr Johnson's talks with the secretary-general, a Downing Street spokeswoman said: "The Prime Minister described Putin's actions in Ukraine as nauseating and blatant aggression, which is having devastating consequences for Ukraine's people.
"They discussed their shared concerns on attacks in areas besieged by Russian forces, such as Mariupol and Kherson, and the need to secure a ceasefire, facilitate humanitarian efforts and allow civilians to leave.
"The Prime Minister made clear the importance of global solidarity with Ukraine, with the UK continuing to work closely with international partners to support Ukraine to defend itself." Britain has positioned itself as one of Kyiv's closest allies both in the run-up to and during the invasion by Russia.
As a result, London has been keen to push for Western capitals to do more to back Ukraine's military as they look to hold back the onslaught from Russian president Vladimir Putin's forces.
Conservative Party chairman Oliver Dowden urged France and Germany to step up their own efforts as he warned Mr Putin would "keep on going and going" as he seeks to capture Ukrainian soil in the east and the south.
Calling for more support from NATO allies, the Cabinet minister told Sky News' Sophy Ridge On Sunday programme: "I think we've had very positive noises from France and Germany but I would like to see further action from them."
He said Mr Johnson's call last week with world leaders, including US president Joe Biden and French president Emmanuel Macron - who looked on course for a second term on Sunday following election exit poll results in France - was part of an effort to lobby for more military backing.
While the West has funnelled military equipment to Ukraine, Mr Zelensky has stressed repeatedly that the country needs more heavy weapons, including long-range air defence systems, as well as warplanes.
It was a message he was expected to impart on US secretary of state, Antony Blinken, and US defence secretary, Lloyd Austin, during their trip to Kyiv on Sunday - the first visit by senior Washington officials since Russia invaded Ukraine 60 days ago.
Bill Browder, a financier and critic of Mr Putin, said Russia's war machine was unlikely to slow down until Europe seriously addressed its dependence on the Kremlin's energy exports.
"Every day Europe - Germany, France, Italy, Austria and various other places that are totally dependent on Russian gas - send Vladimir Putin a billion dollars every day," he told Sophy Ridge. "To put it in really simple terms, this war costs Vladimir Putin a billion dollars a day and every day he gets a billion dollars from Europe and so that's kind of a wash.
"If we really do want to stop this war and we want to stop it from spreading... then we have to stop giving him the money and to stop giving him the money means that Germany is going to have to tighten their belt significantly.
"They are going to have to experience some economic pain."
Most of Sunday's fighting focused on the Donbas in the east, where Ukrainian forces are concentrated and where Moscow-backed separatists controlled some territory before the war. Since failing to capture Kyiv, the Russians are aiming to gain full control over the eastern industrial heartland.
The Russian military reported hitting 423 Ukrainian targets overnight, including fortified positions and troop concentrations, while its warplanes destroyed 26 Ukrainian military sites, including an explosives factory and several artillery depots.
In the south, Russian troops launched fresh airstrikes on a Mariupol steel plant where an estimated 1,000 civilians are sheltering along with about 2,000 Ukrainian fighters.
The Azovstal steel mill where the defenders are holed up is the last corner of resistance in the city, which the Russians have otherwise occupied.
