Boris Johnson used a call with US President Joe Biden and other world leaders to stress the “critical need” for further military support for Ukraine as it faces a Russian offensive in the east.
The Prime Minister told European and north American leaders that sanctions and arms are needed as Vladimir Putin launches a “major Russian offensive” in the Donbas region.
France’s Emmanuel Macron, Germany’s Olaf Scholz, Canada’s Justin Trudeau, Nato secretary-general Jens Stoltenberg and the European Commission leader Ursula von der Leyen were among other leaders on the call.
Downing Street said Mr Johnson updated them on his visit to Kyiv earlier this month, and added: “He underscored the critical need for further military support to Ukraine in the face of a major Russian offensive in the Donbas and ongoing attacks elsewhere.
“The leaders agreed to work together to find a long-term security solution so that Ukraine could never be attacked in this way again. They discussed the need to increase the pressure on Russia with more sanctions against Putin’s war machine, as well as further diplomatic isolation.
“The Prime Minister welcomed President Biden’s leadership, and the allies agreed to work closely together in the weeks and months to come.”
Earlier in the day, Mr Johnson told the weekly meeting of the Cabinet that Russian President Vladimir Putin had been angered by the defeats inflicted on his troops but remained “determined to claim some sort of victory regardless of the human cost”.
Ministers were briefed by a senior national security official who said the next phase of the war – focusing on the Donbas region in eastern Ukraine – was likely to be “an attritional conflict” which could last “several months”.
In his nightly address on Monday, President Volodymyr Zelensky said Russian troops had begun the battle for Donbas – which is part-held by pro-Moscow separatists – for which they had been preparing “for a long time”.
“A very large part of the entire Russian army is now focused on this offensive,” he said.
The Prime Minister’s official spokesman said the national security official had said Russia would seek to exploit its advantage in troop numbers, but experience had shown this was “unlikely to be decisive on its own”.
“There were some signs that Russia had not learned lessons from previous setbacks in northern Ukraine and there was evidence of troops being committed to the fight in a piecemeal fashion,” the official said according to a readout by Mr Johnson’s spokesman.
“Reports of poor Russian morale continue with claims of some Russian troops and even units refusing to fight”.
The Chief of the Defence Staff Admiral Sir Tony Radakin said the UK was continuing play a leading role in supplying military equipment to Kyiv, including in sourcing equipment from other countries which could be used in Ukraine’s defence.
Earlier, it emerged that the UK is to send armoured anti-aircraft vehicles to the Ukrainian military as they prepare for a Russian onslaught.
The Stormer vehicle launches Starstreak anti-aircraft missiles which can be used to target planes and helicopters.
It comes after Britain invited Ukrainian generals to Salisbury Plain earlier this month to see the military kit which could be available to them, including armoured vehicles.
The provision of Stormers – as reported by The Sun – has not yet been officially confirmed, but Defence Secretary Ben Wallace is expected to update MPs this week.
A defence source said: “It is no secret that the UK has committed to helping Ukraine with its anti-air capabilities.
“The Defence Secretary will be making a statement to Parliament this week.”
Earlier, Northern Ireland Secretary Brandon Lewis refused to be drawn on suggestions of a possible prisoner exchange when asked about reports of Britons captured while fighting for the Ukrainian military.
In footage broadcast on Russia’s Rossiya 24 on Monday, Shaun Pinner addresses the Prime Minister and appears to ask for himself and fellow British prisoner of war Aiden Aslin to be swapped for pro-Kremlin politician Viktor Medvedchuk, who has been held in Ukraine.
Mr Lewis told BBC Breakfast: “People should not be taking what is in reality an illegal route through into a very dangerous situation.
“I hope you can appreciate I’m not going to comment on the situation on the two individual cases that you reference.
“But we really should be very clear with people, that isn’t what they should be doing. There are ways to support Ukraine and that is the official ways on the Government website that people can follow.”
For Labour, shadow attorney general Emily Thornberry said “we should be negotiating with the Russians to try to get them back”, but “I don’t think that we can give in to blackmail”.
“We need to make it clear that we don’t negotiate and give in to blackmail when it comes to hostages, and we’ve always said that we need to stick to that,” she told Sky News.