ITV News Correspondent Dan Rivers has the latest on the developing crisis at the border between Ukraine and Russia
Boris Johnson said the international shock of a Russian attack would "echo around the world", as the UK urged Moscow to engage with talks to prevent a conflict.
He told world leaders at the Munich Security Conference that a Russian invasion of Ukraine would spark a “generation of bloodshed and misery" and he called for unity among the West in reacting to any attack.
The prime minister said intelligence shows a conflict between Moscow and Kyiv looks “increasingly likely”, as he vowed to block money linked to the Kremlin from being laundered through the City of London.
Speaking as President Vladimir Putin oversaw drills of his country's nuclear forces on Saturday, Mr Johnson said he did not know what the Russian leader was planning "but the omens are grim".
He told the conference: “If Ukraine is invaded, and if Ukraine is overwhelmed, we will witness the destruction of a democratic state – a country that has been free for a generation with a proud history of elections.”
"Russian parents would mourn the loss of young Russian soldiers, who in their way are every bit as innocent as the Ukrainians now bracing themselves for attack"
Mr Johnson, who visited Kyiv a fortnight ago, warned an incursion by the reported 150,000 Russian troops amassed on the border would lead to parents on both sides "mourning the loss" of young soldiers, with Ukrainians likely to fight for the return of their freedom.
“As I speak to you today, we do not fully know what (Russian) President Putin intends, but the omens are grim and that is why we must stand strong together,” he told the summit.
“I believe that Russia would have absolutely nothing to gain from this catastrophic venture, and everything to lose.
“And while there is still time, I urge the Kremlin to de-escalate, to disengage its forces from the frontier and to renew our dialogue.”
In new signs of fears that a war could start within days, Germany and Austria told their citizens to leave Ukraine and German air carrier Lufthansa cancelled flights to the capital Kyiv, and to Odessa, a Black Sea port that could be a key target in an invasion.
The sounding of the alarm in Germany comes as separatist leaders in eastern Ukraine ordered a full military mobilisation on Saturday and NATO's liaison office in Kyiv said it was relocating staff to Brussels and to the western Ukraine city of Lviv.
Meanwhile top Ukrainian military officials came under a shelling attack during a tour of the front of the separatist conflict in eastern Ukraine on Saturday, according to a journalist from The Associated Press. Violence in eastern Ukraine has spiked in recent days as Ukraine and the two regions held by the rebels each accused the other of escalation. Russia on Saturday said at least two shells fired from a government-held part of eastern Ukraine landed across the border, but Ukrainian Foreign Minister Dmytro Kuleba dismissed that claim as “a fake statement.”
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On Friday, US President Joe Biden said Washington has reason to believe Russian forces “intend to attack” Ukraine in the coming days, including targeting the capital Kyiv – a city with a population of 2.8 million people.
Mr Biden told a White House press briefing on Friday he is “convinced” Mr Putin has “made the decision” to move his military across the border, having spent weeks saying he thought the Russian leader was undecided.
In Munich, Mr Johnson said Russia must be “seen to fail” if diplomacy does not succeed in keeping the peace.
The UK’s commitment to making Moscow pay for any military pursuit will involve ensuring it is “impossible” for Russian oligarchs and companies linked to the Russian state to raise finance in London, Mr Johnson said.
“We will open up the Matryoshka dolls of Russian-owned companies and Russian-owned entities to find the ultimate beneficiaries within,” he vowed, in a pledge to clean up so-called “dirty” Russian money in Britain.
Mr Johnson met with fellow leaders, including German chancellor Olaf Scholz, following his address to discuss the tensions on the Ukraine border.
He held talks with Ukrainian president Volodymyr Zelenskyy in Munich, during which he “underscored the UK’s unequivocal support” and condemned recent Russian aggression, including “shelling in Donbass”, according to Downing Street.
Mr Zelenskyy has also called for Russian President Vladimir Putin to meet him and seek a resolution to the crisis and said Russia could pick the location for the talks.
He said: "Ukraine will continue to follow only the diplomatic path for the sake of a peaceful settlement."
However there was no immediate response from the Kremlin.
Foreign Secretary Liz Truss is also expected to speak at the conference but the Ministry of Defence (MoD) confirmed Ben Wallace has cancelled his appearance in order to stay in the UK to “monitor developments in Ukraine”.
The department, which said the Cabinet minister is due to make a parliamentary appearance on Monday, tweeted: “The Defence Secretary has apologised to the conference organisers, but cannot be absent from the MoD at this time.”
It comes after Britain chose to “temporarily” move its embassy in the Ukrainian capital to Lviv, near the border with Poland, as fears grow about a Kremlin-ordered offensive.
While the West is using the Munich summit to send a message to Moscow, Mr Putin was due to watch Russian forces flex their military muscle during massive nuclear drills.
He has pledged to protect Russia’s national interests against what he sees as encroaching Western threats.
But the Prime Minister said Mr Putin only had himself to blame for Nato forces being deployed to the alliance’s eastern flank and closer to Russia’s border, citing the “tension that he has created”.
There is anxiety in the west that mounting instability in the east of Ukraine, where government forces have been fighting pro-Russia rebels since 2014 in a conflict that has killed some 14,000 people, could spiral into a wider war.
The long-simmering separatist conflict could provide the spark for a broader attack, with London and Washington raising the prospect of Russia using a “false flag” operation to give it cover for an offensive.
Denis Pushilin, the head of the pro-Russian separatist government in the Donetsk region, released a statement on Saturday announcing a full troop mobilisation and urging reservists to show up at military enlistment offices.
A similar announcement quickly followed from Leonid Pasechnik, separatist leader in the Luhansk region.
The announcements came as a mass evacuation of women, children and the elderly from the rebel-held territories in the two regions to neighbouring Russia got under way, in what some commentators speculated is part of Moscow’s efforts to paint Ukraine as the aggressor instead.
Friday marked a day of rising violence that included a humanitarian convoy hit by shelling and a car bombing in the eastern rebel-held city of Donetsk outside the main government building, while two explosions occurred in rebelled-controlled Luhansk.