Vladimir Putin has ordered that Russia’s nuclear deterrent forces are put on high alert.
Mr Putin said aggressive statements by NATO leaders and economic sanctions against Moscow were behind the decision.
Speaking on state television on Sunday, he said: “As you can see, not only do Western countries take unfriendly measures against our country in the economic dimension – illegitimate sanctions that everyone knows about.
Ukraine says ‘4,300 Russian troops have been killed or injured’; follow live updates
“But also the highest-ranking officials of leading NATO countries are allowing themselves to make aggressive statements in relation to our country.
“For this reason I order the minister of defence and the chief of general staff to put deterrent forces on special combat duty.”
‘Incredibly dramatic, escalatory move’
The move will heighten fears across the West that Russian forces could use nuclear weapons in the conflict with Ukraine.
Sky News Moscow correspondent Diana Magnay described the decision as an “incredibly dramatic, escalatory move”, which will “really, really worry people”.
“This is the first time that we’ve ever had a situation, in Putin’s reign at least, where he has put his nuclear forces ready essentially for combat duty, on high alert,” she said.
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Ukraine’s President describes a ‘brutal night’ on the ground
Meanwhile Ukrainian President Volodymyr Zelenskyy sparked hopes of a diplomatic solution to the crisis by agreeing to talks with the Russians.
He had initially rejected the offer of negotiations in Minsk due to Belarus’s strategic links with the Kremlin.
But he has now agreed to talks on the border of the two nations, his office said on Sunday.
Thousands turned out for Ukraine solidarity demonstrations in Berlin. Pic: AP
Protests in Russia and around the world
It comes after Ukrainians spent a third night largely underground sheltering from Russian forces following the initial invasion on Thursday.
In cities in Russia and other countries across the world protests took place against the invasion on Sunday.
Late on Saturday, G7 leaders finally agreed to ban Russian banks from the SWIFT international payment system.
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Kyiv mayor Vitali Klitschko tells the Russians to go home
On the ground in Ukraine, mayor of Kyiv Vitali Klitschko said Russian troops were still around 20 miles from the city centre.
There was “heavy fighting” in the second-largest city of Kharkiv overnight and into Sunday, but its regional governor Oleh Sinegubov insisted Ukraine retains “complete control” of it.