Russia has launched more than 80 strikes against Ukrainian targets today, the UK's Ministry of Defence has said.
It also confirmed that Russian ground forces are advancing across the border, as well as from the North, East and South from Crimea.
This morning Russian President Vladimir Putin announced he had begun a 'military operation' in Ukraine.
Follow our live blog on the Russian invasion here
Since, explosions have been heard and felt in some of the country's major cities, including the Ukrainian capital Kyiv.
Putin announced the action during a televised address early on Thursday morning, saying the move was a response to threats from Ukraine.
It's thought that around 40 people have been killed so far, an adviser to Ukraine's president said.
It comes as earlier in the week, Putin declared independence in the Russian rebel-backed regions of Luhansk and Donetsk, located in the eastern Donbas region.
The Ukrainian Minister of Foreign Affairs, Dmytro Kuleba, said that Russia has launched a “full-scale invasion of Ukraine”.
Mr Kuleba said that Ukraine would defend itself against the Russian aggression “and will win”.
Meanwhile Boris Johnson said a "vast invasion" is underway in Ukraine "by land, by air and by sea.."
In a televised address to the nation, he added: "And this is not, in the infamous phrase, some faraway country of which we know little.
"We have Ukrainian friends in this country, neighbours, co-workers. Ukraine is a country that for decades has enjoyed freedom and democracy, and the right to choose its own destiny.
"We and the world cannot allow that freedom just to be snuffed out. We cannot and will not just look away."
Nato Secretary-General Jens Stoltenberg has said any attack by Russia on an alliance member would be regarded as an attack on all.
Ukraine is not part of Nato, but boarding countries Romania, Slovakia and Poland are.
Speaking at Nato headquarters in Brussels, Mr Stoltenberg said: “Nato’s core task is to protect and defend all allies. There must be no room for miscalculation or misunderstanding.
“An attack on one will be regarded as an attack on all. This is our collective security guarantee.”
Meanwhile, Putin has warned other countries that any attempt to interfere with Russian action would lead to “consequences they have never seen”.
