Russia’s invasion of Ukraine is a “catastrophe for our continent”, Prime Minister Boris Johnson has said.
He chaired an emergency Cobra meeting earlier and will give a televised statement later on the UK’s response to Russia’s “unprovoked attack”.
Mr Putin has insisted he does not plan to occupy Ukraine, but Ukraine has branded it a “full-scale invasion”.
Mr Johnson will outline new sanctions against Russia in the Commons at 17:00 GMT.
He said on Twitter he would also speak to his fellow G7 leaders and called for an urgent meeting of all Nato leaders as soon as possible.
Martial law – which means the military takes control temporarily – has been imposed across the whole of Ukraine and traffic jams built up as people fled the capital city of Kyiv.
In a phone call with Ukrainian President Volodymyr Zelensky early on Thursday morning, Mr Johnson vowed the West “would not stand by as President Putin waged his campaign against the Ukrainian people”.
Mr Johnson had tweeted earlier: “President Putin has chosen a path of bloodshed and destruction by launching this unprovoked attack on Ukraine.”
Foreign Secretary Liz Truss said she had summoned the Russian ambassador to explain Russia’s “illegal, unprovoked invasion of Ukraine” and that the UK would be “imposing severe sanctions”.
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 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 are known to have been killed by Russian shelling, including civilians. A Ukrainian presidential adviser said that more than 40 soldiers had died and dozens more were wounded, but this has not been independently confirmed.
It comes after President Putin announced a “special military operation” in Ukraine’s Donbas region in a televised address to the Russian people 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,” he told the BBC 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.
He said the UK’s “baffling” decision to hold back sanctions until after Russia’s incursion into Ukraine had not deterred Mr Putin.
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 told BBC Breakfast: “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”.
Writing on Twitter, 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 modernise”.
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.