Sanctions and "aggressive statements" from NATO member states prompted the escalation, claimed Putin.
Russia placed its nuclear arsenal -- but also other heavy weapons, such as Kinzhal and Zircon hypersonic missiles -- on "special" alert, the highest form of combat readiness for these units.
President Vladimir Putin ordered the ministry of defence to issue the alert on Sunday, citing sanctions and "aggressive statements" from NATO member states.
European Commission President Ursula von der Leyen announced that the EU will buy and deliver weapons and equipment to Ukraine in what she said was "a watershed moment" on Sunday evening.
More sanctions on top of the ones announced by the bloc on Saturday include "a prohibition on all Russian-owned, Russian registered or Russian-controlled aircraft" as well as an EU-wide ban of "the Kremlin's media machine", including RT and Sputnik.
Additional sanctions were doled out against the Belarusian regime of Alexander Lukashenko, who von der Leyen said was "complicit with the vicious attack".
Meanwhile, the office of the Ukrainian President Volodymyr Zelenskyy said in a statement that he agreed to send in a team of negotiators to meet with their Russian counterparts near the border with Belarus after assurances from Minsk.
More than 200 civilians were reported dead in the war in Ukraine, a defence official said, as heavy fighting was reported in the country's second-largest city Kharkiv.
A regional governor later claimed that the city was back under Ukrainian control.