President Putin has accused the West of engaging in “nuclear blackmail” against Russia and said he has “many weapons to respond”.
In a rare address to the nation, he said he was not bluffing and would use “every means at our disposal” if Russia’s territorial integrity was threatened.
Putin also ordered an immediate “partial mobilization” – calling up the country’s military reserves – a move Russia’s foreign minister said amounted to about 300,000 troops.
President Putin also approved referendums in four regions of Ukraine currently under Russian occupation.
Donetsk, Luhansk, Kherson and Zaporizhzhia announced plans for referendums on Tuesday.
Live: Putin orders ‘partial mobilization’ in Ukraine and calls up military reservists
They are scheduled to take place from September 23 to 27. Together, the regions account for around 15% of Ukrainian territory.
Ukrainian Foreign Minister Dmytro Kuleba has rejected the plans, saying: “The Russians can do whatever they want. Nothing will change.”
The UK Ministry of Defense said the referendums were likely “driven by fears of an imminent Ukrainian attack and the expectation of greater security after formally becoming part of Russia”.
A Ukrainian counterattack in recent weeks has recovered large areas of territory.
Foreign Secretary Gillian Keegan told Sky News that Putin’s threat was something that “had to be taken very seriously because, you know, we’re not in control – I’m not sure he is either in control, really.
“This is obviously an escalation,” he said.
In his speech on Wednesday morning, Putin again repeatedly referred to Ukrainian forces as “neo-Nazis” and accused them of carrying out “acts of terror” against citizens in Russian-controlled areas.