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The Chief of the Defence Staff, Admiral Sir Tony Radakin, told the Cabinet that Britain’s support was moving to a “new phase” as the Ukrainians sought to retake territory captured by the Russians.
The prime minister’s spokesman said “all possible options” were being analysed to ensure the Ukrainians had the equipment they needed while avoiding any “escalatory effects”.
Mr Johnson told the weekly meeting of senior ministers at No 10 Downing Street that, while the Russian campaign continued to fail, Mr Putin could inflict further damage in an attempt to “twist the knife”.
In call on Tuesday, the Prime Minister and other leaders of the “Quint” – the US, France, Germany and Italy – reaffirmed their commitment to supporting the government in Kyiv while reducing their dependence on Russian oil and gas.
Earlier, ministers were briefed by the chairman of the Joint Intelligence Committee, Sir Simon Gass, who said the Russian campaign continued to be beset by poor morale as members of the regime sought to apportion blame for its failures.
Overnight, British military intelligence said Russia was expected to send 1,000 mercenaries from the notorious Wagner Group to join combat operations in eastern Ukraine where they continued to suffer heavy casualties.
The private military company – which is already the subject of UK sanctions – had already reportedly been given the task, so far unsuccessfully, with tracking down and assassinating President Volodymyr Zelenskyy.
Western officials said there appeared to be an “element of desperation” about the move.
“They have a fearsome reputation but that is often for not adhering to the rule law and the law of armed conflict,” one official said.
“If you are reaching for 1,000 Wagner troops thinking that is going to be crucial to succeeding in operations in the Donbas, it should give pause for thought as to how capable your broader force is.”
Meanwhile, Russian and Ukrainian negotiators met in Istanbul for their first face-to-face talks in two weeks, in a further attempt to establish sufficient common ground to end the fighting.
Western officials reacted with scepticism to Russia’s announcement that it was “fundamentally” scaling back operations around Kyiv to “increase mutual trust” in the talks process.
“Nothing that we have seen so far has demonstrated to us that President Putin and his colleagues are particularly serious about that. It is more of a tactical exercise playing for time,” one official said.
It come after an announcement last week by Moscow that it was focusing efforts on “liberating” the Donbas region of eastern Ukraine, currently part-held by pro-Russian separatists.
“Even if they do do what they say they are going to do that is not in any shape or form a cessation of hostilities,” the official said.
“I think we can continue to see as they refocus efforts on the Donbas, if that’s what they do, continued death and destruction there.”
Among those present at the talks in Istanbul in an apparently unspecified role was Chelsea FC owner Roman Abramovich, a long-standing associate of Mr Putin.
His appearance followed reports overnight that he and two Ukrainian negotiators may have been poisoned during an earlier round of talks.
The Bellingcat investigative website said the three suffered eye pain and skin irritation consistent with chemical poisoning after attending peace talks on March 3.
There was speculation that he may have been targeted by hardliners in Moscow after attempting to negotiate humanitarian corridors.
Kremlin spokesman Dmitry Peskov dismissed the claims, saying they “do not correspond to reality,” but Downing Street described the allegations as “very concerning”.
“We’re working with international partners to establish more information,” Mr Johnson’s official spokesman said.
The spokesman did not rule out reports that the UK, along with the US and Turkey, could act as guarantors of Ukrainian security if it agreed to neutrality as part of any peace settlement.
“As far as I am aware there has been no formal request for that,” the spokesman said.
“We would listen very carefully for any request of UK involvement in a long-term peaceful solution but there has been nothing formal on that front.”