The White House is accusing Russia of escalating the crisis in Ukraine after President Vladimir Putin put his country's nuclear deterrence troops on standby.
The first talks between Russia and Ukraine are due to take place later today, but the European Union and the US have unveiled additional sanctions measures in a bid to cripple Russia's economy.
Our Washington Correspondent Simon Marks reports.
IN: "The White House…."
OUT: "…possible nuclear clash"
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