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Any incursion into Ukraine by Russia would be a "massive strategic mistake" that would "come at a severe cost" to Moscow, the foreign secretary has told MPs.
"We stand with our friend against hostile actors and we will defend democracy at the frontier of freedom in eastern Europe and around the world," she said.
"Any Russian military incursion into Ukraine would be a massive strategic mistake and would come at a severe cost."
'Russia is the aggressor here'
A build-up of an estimated 100,000 Russian troops and military equipment on the Ukrainian border has sparked fears Moscow is preparing to invade the country.
The crisis comes seven years after Russian forces marched into Crimea and seized the territory from Ukraine.
The foreign secretary said Russia was waging a campaign to "subvert its democratic neighbours" accompanied by "baseless rhetoric and disinformation".
"They have falsely cast Ukraine as a threat to justify their aggressive stance. They falsely accuse NATO of provocation - this could not be further from the truth," she said.
"Russia is the aggressor here. They have massed a huge number of troops along the Ukrainian border and in illegally annexed Crimea.
"There is no justification whatsoever for Russia's bellicose stance towards Ukraine. It is unprovoked and it is part of a wider pattern of behaviour by the Kremlin, reliant on disinformation and mistrust to seek to gain an upper hand."
Truss warns of 'crucial moment'
The foreign secretary said she will be joining a meeting of NATO foreign ministers on Friday to discuss the crisis, while talks between senior US and Russian officials in the Swiss city of Geneva will take place next week.
Ms Truss said these discussions would be "absolutely critical for peace and security in Europe".
"It's vital that NATO is united in pushing back against Russia's threatening behaviour, and together we must hold Russia to its longstanding obligations," she told MPs.
"There can be no rewards for aggression.
"We are reaching a crucial moment. The only way forward is for Russia to de-escalate and pursue a path of diplomacy.
"We will continue to stand together with our allies, steadfast in support of Ukraine and its future as a free and sovereign democracy."
Washington and Moscow talks
Vladimir Putin and Joe Biden spoke on the phone for 50 minutes at the end of last month, with the Kremlin saying that the Russian leader warned the US president that new sanctions would be a big mistake.
White House spokesperson Jen Psaki said Mr Biden "urged Russia to de-escalate tensions with Ukraine" during the call.
Moscow's security demands, submitted last month, included denying NATO membership to Ukraine and other former Soviet countries and the rolling back of Western military deployments in Central and Eastern Europe.
The US has also repeatedly made clear that Russia has no say in NATO's composition or the promise of future NATO admission made to Ukraine and Georgia in 2008.
But Washington and its allies have agreed to hold talks with Moscow to discuss its concerns.
Mr Putin has said he will consider a range of options if the West fails to provide the guarantees he is seeking.