Russian president Vladimir Putin has placed his country's nuclear deterrent systems on a "special regime of combat duty", reports suggest.
The news comes after he ordered his forces to invade Ukraine, prompting various sanctions from western nations, including the UK.
Putin told state TV that NATO member states made "aggressive statements" and imposed hard-hitting financial sanctions, during a broadcast on Sunday, February 27.
He ordered the Russian defence minister and the chief of the military's general staff to put the nuclear deterrent forces on alert, according to The Mirror.
Putin is said to becoming increasingly frustrated at the slower-than-expected pace of the invasion as Russian troops have so far failed to capture the capital of Kyiv.
Overnight, Russian troops and tanks entered Kharkiv, triggering a fight on the streets for Ukraine's second city.
Speaking on state TV, Putin said: "As you can see, not only do Western countries take unfriendly measures against our country in the economic dimension - I mean the illegal sanctions that everyone knows about very well - but also the top officials of leading NATO countries allow themselves to make aggressive statements with regards to our country."
