NATO is said to be preparing troops on the ground despite Vladimir Putin's "nuke threat".
The military alliance has activated defence plans and would boost the alliance’s “deterrence and defence” by adding to land, sea and air forces on its eastern flank.
That's as well as reportedly putting some 100 warplanes on high alert and comes after Russian forces entered Chernobyl to seize the nuclear plant.
World leaders have condemned President Putin for invading Ukraine, with Prime Minister Boris Johnson saying "diplomatically, politically, economically – and eventually, militarily – this hideous and barbaric venture of Vladimir Putin must end in failure".
In a chilling video, President Putin appeared to threaten nuclear strikes should the West try to attack Russia in retaliation.
NATO secretary-general Jens Stoltenberg said its plans allow military commanders to move forces.
"Peace on our continent has been shattered," he said as part of a news conference.
"Russia is using force to try to rewrite history, and deny Ukraine its free and independent path."
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He later added: "We must respond with renewed resolve and even stronger unity.
"What we do is defensive."
Altogether, NATO's Response Force (NRF) comprises of around 40,000 troops and is already on heightened alert.
The alliance is set to hold an emergency summit of its 30 member nations on Friday.
The Russian President said the “consequences” of any attempt to strike back following the country's invasion of Ukraine would provoke a response “never seen in history”.
President Putin's chilling address was a stark reminder to the West of Russia's nuclear power.
This afternoon, Russian forces entered Chernobyl to seize the nuclear plant amid a firefight with Ukrainians.
The President of Ukraine took to Twitter to say Russian forces were trying to seize the zone.
"Russian occupation forces are trying to seize the #Chornobyl_NPP," tweeted Volodymyr Zelenskyy.
"Our defenders are giving their lives so that the tragedy of 1986 will not be repeated. Reported this to @SwedishPM.
"This is a declaration of war against the whole of Europe."
The United Nations refugee agency said on Thursday that the situation in Ukraine was quickly deteriorating after Russia's invasion and pleaded with neighbouring countries to keep their borders open to people seeking protection.
Ukrainian Ambassador to the UK Vadym Prystaiko said Ukraine is "putting up a real fight" but added some places are proving difficult to defend.
"We are defending our land," he said.
Boris Johnson condemned Russia for unleashing a "tidal wave of violence" against Ukraine without provocation or credible excuse, labelling the invasion as an "attack on democracy and freedom".
In an earlier statement, NATO said it had decided "to take additional steps to further strengthen deterrence and defence across the alliance".
It added that its "measures are and remain preventive, proportionate and non-escalatory".
The Ukrainian ambassador to the UK has called for NATO forces to fight on Ukrainian soil.
Vadym Prystaiko, who use to be ambassador of Ukraine to NATO, said he personally wanted to see allied forces fighting in Ukraine against the Russian invasion.
He also called for a list of strong sanctions, to target the Swift payment system and Russian central bank.
The Mayor of Kyiv has also ordered a curfew in the city amid the Russian invasion, with citizens trying to flee or take shelter.
Vitali Klitschko, the former boxer, made the announcement this afternoon, according to news agency Tass.
Kyiv residents have reportedly been told to shelter as helicopters fly overhead.
