The former president of Russia has warned that World War Three could break out if any Nato member encroaches on the disputed land of Crimea.
Moscow has launched a wave of fresh missile attacks on Kyiv in recent days to coincide with the meeting of G7 leaders in Germany.
And former leader Dmitry Medvedev, who served as president between 2008 and 2012, sharpened the rhetoric coming out of the Kremlin on Tuesday.
Medvedev told reporters any advancement on the peninsular in the south of Ukraine - which has been disputed territory for the past eight years - could amount to a declaration of war on Russia.
"For us, Crimea is a part of Russia. And that means forever. Any attempt to encroach on Crimea is a declaration of war against our country," Medvedev told the news website Argumenty i Fakty.
"And if this is done by a Nato member state, this means conflict with the entire North Atlantic alliance; a World War Three. A complete catastrophe."
Medvedev, now deputy chairman of Russia's Security Council, also said if Finland and Sweden joined Nato, Russia would strengthen its borders and would be "ready for retaliatory steps". He warned that could include the prospect of installing Iskander hypersonic missiles "on their threshold."
Watch: Britain and allies face "1937 moment" following Ukraine war, head of British Army warns
In 2014, Russia seized Ukraine's southern peninsula of Crimea, an area of strategic importance in an invasion which marked what one Western intelligence official described as the "creeping militarisation" of the Black Sea.
While Nato and the international community deemed the annexation illegal, they failed to stop it and Moscow has since established two federal "subjects" in the area - the Republic of Crimea and the federal city of Sevastopol.
Vladimir Putin's full-scale invasion of Ukraine in February this year has been seen as an attempt by Moscow to annex more territory and influence in the country.
However, Russian troops faced stiffer-than-expected resistance and have since withdrawn and focused its efforts in the Donbas region in the south east. On Saturday, pro-Russian forces claimed control of the strategically import city of Severodonetsk.
The invasion has increased tensions between Russia and the West, specifically between Nato.
The military alliance announced on Monday it was boosting its high-readiness force nearly eightfold to 300,000 troops as part of its response to an “era of strategic competition”.
The reaction force currently has around 40,000 soldiers who can be deployed quickly if needed.
Nato Secretary-General Jens Stoltenberg called it "the biggest overhaul of our collective defence and deterrence since the Cold War".
More countries are looking at joining Nato following the invasion, with Sweden and Finland each applying to join the bloc.
On Tuesday, the new head of the British army is expected to warn that the UK must be ready to "fight and win", and that these times are "our 1937 moment".
General Sir Patrick Sanders, the Chief of the General Staff, will say he had never seen such a clear threat to peace and democracy as the “brutal aggression” of Putin.
In his first public engagement since taking up his post, Gen Sanders will say his focus is on mobilising the Army to help prevent the spread of war in Europe by being “ready to fight and win alongside our Nato allies and partners”.
“In all my years in uniform, I haven’t known such a clear threat to the principles of sovereignty and democracy, and the freedom to live without fear of violence, as the brutal aggression of president Putin and his expansionist ambitions,” he is expected to say.
“This is our 1937 moment. We are not at war – but must act rapidly so that we aren’t drawn into one through a failure to contain territorial expansion.
“I will do everything in my power to ensure that the British Army plays its part in averting war.”
He will liken the current situation to the run up to the Second World War, saying Britain must be prepared to “act rapidly” to ensure it is not drawn into a full-scale conflict through its failure to contain Russian expansionism.