Prime Minister Boris Johnson said Russian President Vladimir Putin has “chosen a path of bloodshed and destruction” with his attack on Ukraine and that the UK and its allies would respond “decisively”.
It came as Ukraine’s president declared martial law and Russia’s military said it had targeted Ukrainian air bases and other military assets and had not targeted populated areas.
A Downing Street spokesperson said the Prime Minister will chair a Cobra committee meeting at 7.30am to discuss the response to the “horrific attacks”.
In a statement on Twitter, Mr Johnson said he had spoken with Ukrainian President Volodymyr Zelensky to discuss “next steps”.
“I am appalled by the horrific events in Ukraine and I have spoken to President Zelenskyy to discuss next steps,” he said.
“President Putin has chosen a path of bloodshed and destruction by launching this unprovoked attack on Ukraine.
“The UK and our allies will respond decisively.”
A Downing Street spokesperson said Mr Johnson had assured Mr Zelensky the West “would not stand by as President Putin waged his campaign against the Ukrainian people”.
“The Prime Minister said he hoped Ukraine could resist and that Ukraine and its people were in the thoughts of everyone in the United Kingdom people during this dark time,” the spokesperson said.
Mr Putin announced the action during a televised address early on Thursday morning, saying the move was a response to threats from Ukraine.
He said Russia does not have a goal to occupy Ukraine, but the responsibility for bloodshed lies with the Ukrainian “regime”.
He also warned other countries that any attempt to interfere with the Russian action would lead to “consequences they have never seen”.
Mr Putin accused the US and its allies of ignoring Russia’s demand to prevent Ukraine from joining Nato and offer Moscow security guarantees.
He said the Russian military operation aims to ensure a “demilitarisation” of Ukraine, adding that all Ukrainian servicemen who lay down arms will be able to safely leave the zone of combat.
Explosions could be heard in the Ukrainian capital of Kyiv shortly after Mr Putin’s address, while explosions were also reported in the cities of Odesa and Kharkiv.
Defence Secretary Ben Wallace described Russia’s actions as “naked aggression against a democratic country” and said no one had been fooled by the Kremlin’s “false flags and fake narratives”.
“The Russian Federation has today further violated Ukrainian Sovereignty,” he said.
“Despite the efforts of the international communities, Russia has chosen conflict. No one has been fooled by the Kremlin’s false flags and fake narratives.
“This is naked aggression against a democratic country which had dared to express a different aspiration than being a supine neighbour to Russia. No one should forget this day. Putin thinks this land grab is about securing his legacy – it will be, but not the one that he wishes.”
US President Joe Biden denounced Russia’s decision to conduct a military operation in Ukraine as an “unprovoked and unjustified attack”, declaring “the world will hold Russia accountable”.
Mr Biden said on Twitter: “President Zelenskyy reached out to me tonight and we just finished speaking. I condemned this unprovoked and unjustified attack by Russian military forces. I briefed him on the steps we are taking to rally international condemnation, including tonight at the UN Security Council.
“He asked me to call on the leaders of the world to speak out clearly against President Putin’s flagrant aggression, and to stand with the people of Ukraine.
“Tomorrow, I will be meeting with the Leaders of the G7, and the United States and our Allies and partners will be imposing severe sanctions on Russia. We will continue to provide support and assistance to Ukraine and the Ukrainian people.”