Russia's President Vladimir Putin has accused the US of trying to draw his country into a war in Ukraine.
In his first significant comments on the crisis in several weeks, he said America's goal was to use a confrontation as a pretext to impose more sanctions on Russia.
He also said the US was ignoring Russia's concerns about Nato alliance forces in Europe.
Tension is high over a Russian troop build-up close to Ukraine's borders.
Russia has in recent weeks moved about 100,000 troops - equipped with everything from tanks and artillery to ammunition and air power - to Ukraine's border.
But Russia denies Western accusations that it is planning an invasion, nearly eight years after it annexed Ukraine's southern Crimea peninsula and backed a bloody rebellion in the eastern Donbas region.
Moscow in turn accuses the Ukrainian government of failing to implement an international deal to restore peace to the east, where at least 14,000 people have been killed and Russian-backed rebels control swathes of territory.
Meanwhile, Ukrainian President Volodymyr Zelensky warned on Tuesday that a Russian invasion would "not be a war between Ukraine and Russia - this would be a war in Europe, a full-scale one".
Speaking after talks with Hungarian Prime Minister Viktor Orban in Moscow, Mr Putin said: "It seems to me that the United States is not so much concerned about the security of Ukraine... but its main task is to contain Russia's development. In this sense Ukraine itself is just a tool to reach this goal."
Rivalry between Russia and the US, which still possess the world's biggest nuclear arsenals, dates back to the Cold War (1947-89). Ukraine was then a crucial part of the communist Soviet Union, second only to Russia.
Mr Putin said the US had ignored Moscow's concerns in its response to Russian demands for legally binding security guarantees, including a block on Nato's further expansion to the east.
He suggested that if Ukraine were granted its wish to join Nato, it could drag the other members into a war with Russia.
"Imagine that Ukraine is a Nato member and a military operation [to regain Crimea] begins," the Russian leader said. "What - are we going to fight with Nato? Has anyone thought about this? It seems like they haven't."
In Ukraine itself, visiting UK Prime Minister Boris Johnson accused Mr Putin of effectively "holding a gun... to the head of Ukraine" and he called on the Kremlin to step back from a "military disaster".
Speaking after talks with Mr Zelensky in the capital Kyiv, Mr Johnson told reporters the Ukrainian army would fight back in the event of an invasion.
"There are 200,000 men and women under arms in Ukraine," he said. "They will put up a very, very fierce and bloody resistance and I think that parents, mothers, in Russia, should reflect on that fact. And I hope very much that President Putin steps back from the path of conflict and that we engage in dialogue."
Mr Johnson warned that the UK would respond to Russian aggression with a "package of sanctions and other measures to be enacted the moment the first Russian toecap crosses further into Ukrainian territory".
The UK has announced it is giving £88m ($119) to Ukraine to promote stable governance and energy independence from Russia.
Mr Zelensky called for sanctions to be introduced before any escalation, saying he would support any move by the UK to deal with "dirty money" allegedly linked to the Kremlin being laundered through the City of London.
BBC
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