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The UK prime minister will chair an emergency council meeting on Tuesday morning to discuss the latest developments in Ukraine.
The meeting, which is scheduled to take place at 6.30am, will be used to “co-ordinate the UK response”, including agreeing a “significant package of sanctions to be introduced immediately”, according to a Downing Street spokesperson.
Prime Minister Boris Johnson told Ukrainian President Volodymyr Zelenskyy that he believes a Russian invasion is “a real possibility in the coming hours and days”, No 10 said.
It comes after Foreign Secretary Liz Truss said the UK will announce new sanctions against Russia on Tuesday “in response to their breach of international law and attack on Ukraine’s sovereignty and territorial integrity”.
The Kremlin has said President Vladimir Putin is ordering Russian forces to “maintain peace” in eastern Ukraine.
The peacekeeping operation is in two regions, which the Russian president earlier said he would recognise as breakaway republics, reports said.
Mr Johnson had said the decision from Mr Putin to recognise Donetsk and Luhansk was an “ill omen” and a flagrant breach of international law.
A representative said on Monday evening the prime minister would chair a Cabinet Office Briefing Room meeting at 6.30am "to discuss the latest developments in Ukraine and to co-ordinate the UK response including agreeing a significant package of sanctions to be introduced immediately.”
Meanwhile, his Downing Street office said Mr Johnson outlined his “grave concern at recent developments in the region” in a call with the Ukrainian president.
Mr Johnson told President Zelenskyy that he would “explore sending further defensive support to Ukraine” at the request of the country’s government, the prime minister's office said.
Ms Truss tweeted about the new sanctions from the UK on Monday evening after saying Mr Putin’s actions could not be allowed to go “unpunished”.
“Tomorrow we will be announcing new sanctions on Russia in response to their breach of international law and attack on Ukraine’s sovereignty and territorial integrity,” she said.
The foreign secretary went on to say that she spoke to Josep Borrell, the EU’s high representative for foreign affairs, following “Russia’s latest assault on Ukrainian sovereignty”.
“We agreed [the UK] and [the EU] will co-ordinate to deliver swift sanctions against Putin’s regime and stand shoulder-to-shoulder with Ukraine,” she said.
In addition, Ms Truss said she spoke to Ukrainian Foreign Minister Dmytro Kuleba, to give her “full support to Ukraine”, adding that the pair “discussed the sanctions [the] UK will be imposing on Russia together with our allies”.
Earlier, she said the recognition of the self-proclaimed Donetsk People’s Republic and the Luhansk People’s Republic as independent states “demonstrates Russia’s decision to choose a path of confrontation over dialogue”.
“We will co-ordinate our response with allies,” she said.
“We will not allow Russia’s violation of its international commitments to go unpunished.”
The EU also announced it would impose sanctions in response to Russia’s recognition of the two states.
In a joint statement, European Commission president, Ursula von der Leyen, and European Council president, Charles Michel, said it was an “illegal act”.
Nato secretary general Jens Stoltenberg said: “This further undermines Ukraine’s sovereignty and territorial integrity, erodes efforts towards a resolution of the conflict, and violates the Minsk Agreements, to which Russia is a party.”
Emily Ferris, a research fellow specialising in Russia and Eurasia at the Royal United Services Institute defence and security think tank, told the PA news agency: “The troops are likely moving to Donetsk and Luhansk to consolidate Russia’s control over the region and assist in formalising the decision taken earlier by the Russian security council to recognise the republics as ‘independent’ statelets.
“While it’s not clear yet what Putin has decided to do, some senior Kremlin officials have voiced sentiments that Putin might not push the border of the conflict further into the rest of Ukraine.
“Certainly Putin’s address was starkly different to his usually much more controlled public persona, and the tone of his speech was decidedly dark.”