He also warned Moscow's "extensive playbook of aggression" would also be met with action - following his crunch talks with his counterpart foreign minister Sergei Lavrov, aimed at defusing tensions over the former Soviet republic.

With an estimated 100,000 Russian troops amassed on the Ukrainian border, many fear the Kremlin is preparing an invasion, leading to a bloody conflict, although it denies this.

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Speaking during a news conference in Geneva following his meeting, Mr Blinken said the discussion with Mr Lavrov was not a negotiation but "a candid exchange of views".

"I conveyed the position of the US and our European allies and partners that we stand firmly with Ukraine in support of its sovereignty and territorial integrity," he said.

"We have been clear, if any Russian military forces move across Ukraine's border, that is a renewed invasion. It will be met with swift, severe and a united response from the United States and our partners and allies."

The US secretary of state also warned of Russia's "extensive playbook of aggression short of military action", which he said includes "cyberattacks, paramilitary tactics and other means of advancing their interests aggressively without using military action".

"Those types of Russian aggression, will be met with a decisive, calibrated and again united response," Mr Blinken said.

He added: "We didn't expect any major breakthroughs to happen today, but I believe we are now on a clearer path to understanding each other's positions."

On Russia's repeated insistence that it has no plans to invade Ukraine, Mr Blinken said: "We're looking at what is visible to all, and it is deeds and actions and not words that make all the difference."

But he said talks would continue and that the US was open to a meeting between the Russian leader Vladimir Putin and US President Joe Biden, if it would be "useful and productive".

Mr Lavrov described the talks in Geneva as "constructive and useful" and said the US had agreed to provide written responses to Russian demands on Ukraine and NATO next week.

But he said: "I can't say whether we are on the right path or not.

"We will see when we get the American responses."

Mr Lavrov also said he hoped "emotions would cool down" over Ukraine and repeated the claim that Russia posed no threat to its former Soviet neighbour.

Moscow has demanded that NATO promise Ukraine will never be allowed to join the military alliance.

It also wants the allies to remove troops and military equipment from parts of eastern Europe.

However, the US and its allies have rejected these demands and repeatedly promised "severe" consequences including tough economic sanctions - though not military action - against Russia if an invasion goes ahead.

It came as the US treasury department slapped new sanctions on four Ukrainian officials, accused of being at the centre of a Kremlin effort to damage Ukraine's ability to "independently function".

Russia seized control of Ukraine's Crimea peninsula in 2014 and backed a separatist insurgency in the eastern part of the country, that has claimed more than 14,000 lives.

Mr Putin has faced limited international consequences for those moves, but the West says a new invasion would be different.

Mr Blinken has also sought to stress US unity with its partners after US President Joe Biden drew widespread criticism for saying retaliation for Russian aggression in Ukraine would depend on the circumstances and that a "minor incursion" would see a lesser response.

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Mr Biden subsequently sought to clarify his comments by cautioning that any troop movements across Ukraine's border would constitute an invasion and that Moscow would "pay a heavy price" for such an action.

Moscow has denied it is planning an offensive and instead accused the West of plotting "provocations" in Ukraine, citing the supply of thousands of anti-tank weapons by the British military, in the face of the Kremlin troop build-up.