Vladimir Putin has threatened a “military-technical response” if Western countries continue with what he calls “unfriendly” actions over Ukraine.
Speaking to military officials, Mr Putin said there was no room to retreat in the standoff currently centred on the former Soviet state.
Concerns are growing that Russia is planning to invade its neighbour with thousands of troops gathering near the border.
Russia has denied it is preparing a military advance.
What Putin said – and what is being threatened
Speaking on Tuesday, Mr Putin said: “What the US is doing in Ukraine is at our doorstep… And they should understand that we have nowhere further to retreat to. Do they think we’ll just watch idly?
“If the aggressive line of our Western colleagues continues, we will take adequate military-technical response measures and react harshly to unfriendly steps.”
The exact measures the 69-year-old leader wants to use were not elaborated upon.
However the deputy foreign minister, Sergei Ryabkov, previously threatened that Russia could redeploy intermediate-range nuclear missiles in Europe.
Mr Putin last week told French President Emmanuel Macron that he wanted to prevent an eastern expansion of NATO.
What is Russia unhappy about
Russia says NATO is planning to send intermediate-range nuclear missiles of its own to Ukraine.
It comes as Ukraine continues to angle towards a friendlier relationship with the Western powers.
Other former Soviet nations, like Lithuania – which has just agreed a $125m weapons deal with the US – are also angling more towards the West.
Ukraine’s president, Volodymr Zelenskiy, said on Friday that he was willing to meet with Russia for “direct talks” – but this was rejected because Moscow says it needs a clear agenda for the meeting.
In a statement, the Kremlin said Mr Macron was told another summit of the Normandy Group – France, Germany, Ukraine and Russia – would be contingent on commitments from Ukraine to abide by existing peace agreements.
Kiev, conversely, says it is Russia and its allies which are refusing to engage.
What has the West said
The new chancellor of Germany, Olaf Scholz, told Mr Putin about his concerns over the Ukrainian situation in a phone call, Berlin said on Tuesday, and called for a de-escalation.
Prime Minister Boris Johnson told Mr Putin last week that an invasion of Ukraine would be a mistake – later telling MPs that a “tough package of economic sanctions” by the UK and its allies would be implemented.
US Secretary of State Antony Blinken said on Tuesday that talks between President Joe Biden and Mr Putin were off the cards for the moment.
“I think we have to see if, in the first instance, there’s any progress diplomatically,” he said.
Karen Donfried, the US’s top diplomat for Europe, said: “As President Biden has told President Putin, should Russia further invade Ukraine, we will provide additional defensive materials to the Ukrainians above and beyond that which we are already in the process of providing.”