Boris Johnson is expected to discuss the deepening crisis in Ukraine with Joe Biden and other G7 leaders after Russia further fanned fears it is trying to create a pretext for war.
Kyiv declared a nationwide state of emergency and was braced for an all-out invasion following a Kremlin declaration that rebel leaders in eastern Ukraine had requested military assistance.
The White House said the plea from Russian-backed separatists was an example of the “false-flag” operation the West was expecting Vladimir Putin would use to strike.
Under increasing pressure to go further on sanctions against those supporting the Russian president, Mr Johnson is expected to join a virtual meeting of G7 leaders on Thursday.
Washington confirmed the US president will participate in the call after Mr Biden increased his economic action against Moscow by targeting measures against the company that built the Nord Stream 2 gas pipeline to Germany.
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Ukrainian President Volodymyr Zelensky made an emotional televised address in Russian after a 30-day state-of-emergency order handing the government sweeping powers in the interest of national security was approved.
“The people of Ukraine and the government of Ukraine want peace. But if we come under attack that threatens our freedom and lives of our people we will fight back,” he said, as he rejected Moscow’s claim Ukraine is a threat to Russia.
Following Mr Zelensky’s speech, the UN Security Council scheduled an emergency meeting at the request of Ukraine, which said there is an immediate threat of a Russian invasion.
It came as the Ukrainian government began closing airports in eastern Ukraine until at least 7am (9am London) because of the expected confrontation.
Mr Johnson has vowed the UK will send more lethal defensive weapons and other aid to bolster Ukraine’s resistance to a fully-fledged Russian offensive.
Downing Street cited “operation security reasons” in not detailing what equipment would be dispatched, but Britain has previously sent anti-tank firepower to Ukraine.
Mr Putin’s internationally-criticised decision to recognise the two breakaway states of Donetsk and Luhansk has led to co-ordinated economic sanctions from the UK, US and EU.
Britain has targeted three Oligarchs close to Putin and five smaller banks in what Mr Johnson called a “first barrage” of sanctions on Tuesday.
The Prime Minister said the next wave of measures “will stop all Russian banks, all oligarchs, all Russian individuals, raising money on London markets”.
But Labour leader Sir Keir Starmer is among those calling for him to go further now to include barring Russians from the Swift financial system and ban trading in Russian sovereign debt.
Senior Conservative MP Tobias Ellwood, the chair of the Commons Defence Committee, said the existing strategy is “simply baffling”.
“We’re holding back any sanctions until after an invasion, as if somehow the tanks will then turn around and go home,” he told BBC Newsnight.
“I have to say this is just feeble. If Kyiv falls, history will ask why the West did not do more.”
Russia is currently estimated to have amassed around 150,000 troops on Ukraine’s border, with warplanes and armoured vehicles continuing to deploy in the area.
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Mr Putin has said the only way to resolve the crisis is for the government in Kyiv to give up its ambitions to join Nato and to accept the “demilitarisation” of the country.
Defence Secretary Ben Wallace said the Russian leader had gone “full tonto” and has been left with no allies over his actions in Ukraine.