The UK will respond decisively to Russia’s “unprovoked attack” on Ukraine, Boris Johnson has said.
The prime minister chaired an emergency Cobra committee meeting earlier after President Putin launched a military assault on the country.
Mr Putin has insisted he does not plan to occupy Ukraine, but Ukraine has branded it a “full-scale invasion.
Mr Johnson will address the nation later and reveal new sanctions against Russia in the Commons at 17:00 GMT.
In a phone call with Ukrainian President Volodymyr Zelensky, Mr Johnson vowed the West “would not stand by as President Putin waged his campaign against the Ukrainian people.
Mr Johnson wrote that “President Putin has chosen a path of bloodshed and destruction by launching this unprovoked attack on Ukraine.
Labour leader Sir Keir Starmer warned Putin’s attack on Ukraine would “have horrendous and tragic consequences that will echo throughout the world and throughout history”, and called for the “hardest possible sanctions” against the Russian leader and his associates.
He added: “There can be no space for equivocation when faced with the evil that Putin has unleashed.
His actions pose a grave threat to the international order on which we all depend.
Russian forces have launched an assault on Ukraine, with reports of explosions near major cities across the country early on Thursday morning.
Ukraine has declared a month-long state of emergency, as Russia said it had carried out air strikes on Ukraine’s military infrastructure and border guard units but not targeting populated areas.
Russian military vehicles are reported to have breached Ukraine’s border in a number of places, in the north, south and east, including from Belarus.
At least seven people have been killed by Russian shelling, while another 19 are missing, Ukraine has said.
Warning sirens were heard blaring across the capital Kyiv which is home to almost three million people as traffic queued to leave the city during the night and crowds sought shelter in the underground.
President Putin announced a “special military operation” to the Russian people in Ukraine’s Donbas region earlier and said the Ukrainian people would be able “to choose freely” who ran the country.
Mr Putin also urged Ukrainian soldiers in the combat zone in the east of the country to lay down their weapons and return to their homes, warning that Moscow’s response would be “instant” if anyone tried to take on Russia.
On Wednesday, the UK announced a package of sanctions against Russia as part of a co-ordinated Western response to the crisis.
Mr Johnson announced that five Russian banks had had their assets frozen and three Russian billionaires would have travel bans imposed.
It followed the Kremlin ordering troops into the rebel-held Ukrainian regions of Donetsk and Luhansk on recognising them as independent.
Foreign Office minister James Cleverly warned Mr Putin’s comments in recent days suggested he wanted to create “a wider Russian empire in all but name.
“We do fear that this is the start of quite an extensive military operation.
This is not just limited to the east.
There have been military strikes across a number of parts of Ukraine,” Mr Cleverly said on Thursday morning.
He said new UK sanctions against Russia would be announced “in waves” and warned it would be “the largest and most severe economic sanctions” package Russia had ever seen.
Asked about the prospect of UK boots on the ground in Ukraine, Mr Cleverly said that while Ukraine was a “good friend” of the UK, it was not a member of the Nato military alliance which obliges members to come to one another’s aid in the event of an armed attack against a member state.
’’The UK is providing additional military support to eastern Nato member states and will support Ukrainians in their defence of their homeland,’’ he added.
UK nationals were advised two weeks ago to leave Ukraine while commercial flights were still available.
Those remaining have again been urged to leave Ukraine immediately if they judge it is safe to do so.
Ukraine has closed its airspace to civilian flights.
The only airlines still flying from the UK, Wizz Air and Ryanair, have suspended all flights to the country.
Tobias Ellwood, chairman of the defence select committee, said the West had failed to alter the trajectory of the Russian invasion and accused the UK of dithering over the crisis.
He said the UK’s “baffling” decision to hold back sanctions until after Russia’s incursion into Ukraine had not deterred Mr Putin.
“If Kyiv falls, history will ask why the West did not do more and why Nato’s hard power deterrents the one asset Putin fears was not better harnessed.
He said Ukraine needed tangible support including weapons and communication systems, counter cyber capabilities and that a Nato no-fly zone would be a “gamechanger” in deterring Russia.
Tom Tugendhat, chairman of the Foreign Affairs Committee, described the scenes of people abandoning their homes in Ukraine as “a human tragedy.
He said “To see these columns of people forced out by naked aggression by Russia and by Vladimir Putin is horrific and extremely painful, and we should be thinking very clearly about the families torn apart, the lives separated.
The latest move by Russia has drawn international condemnation, with US President Joe Biden denouncing Russia’s military action as an “unprovoked and unjustified attack”, declaring “the world will hold Russia accountable.
He added the US and its allies and partners would impose “severe sanctions on Russia” and continue to provide support to Ukraine and its people.
European Commission President Ursula von der Leyen accused Mr Putin of “bringing war back to Europe” and warned a raft of “massive” sanctions would be proposed later aimed at Russia’s economic base and its “capacity to modernize.
Nato Secretary General Jens Stoltenberg criticised Russia’s “reckless” attack on Ukraine and said despite weeks of tireless international diplomacy, Russia had chosen “the path of aggression.
Mr Stoltenberg has called an emergency meeting of the North Atlantic Council this morning.
A news conference is expected to be held in Brussels at 12:00 local time.
Defence Secretary Ben Wallace branded Russia’s actions “naked aggression against a democratic country” and said no one had been fooled by the Kremlin’s “false flags and fake narratives.
Fears of a Russian invasion into Ukraine have been rising for months.
Mr Putin has accused the West of ignoring Russia’s demands to prevent Ukraine from joining the western Nato military alliance and offer Moscow security guarantees.
UK condemns unprovoked Russian attack on Ukraine