President Barack Obama has stepped up the rhetoric against Russian intervention in Ukraine calling on Moscow to "pull back its troops" immediately.
Mr Obama urged Moscow to reduce the number of Russian troops deployed on the Ukrainian border and acknowledged that, while it may be "an effort to intimidate" Kiev, Russia may also have "additional plans" in the region.
"You've seen a range of troops massing along that border under the guise of military exercises," he told CBS This Morning.
"But these are not what Russia would normally be doing, and it may be simply be an effort to intimidate Ukraine, or it may be that they've got additional plans."
He added: "In either case, what we need right to resolve and de-escalate tensions is for Russia to move back those troops and begin negotiations."
Mr Obama said President Vladimir Putin had shown a "deeply-held grievance about what he considers to be the loss of the Soviet Union", but warned against actions that could "revert back" to the Cold War.
He said: "There's a strong sense of Russian nationalism and a sense that somehow the West has taken advantage of Russia in the past.
"What I have repeatedly said is that he may be entirely misreading the West. He's certainly misreading American foreign policy."
Meanwhile ousted president Viktor Yanukovych has called for a nationwide referendum to determine the "status of every region in Ukraine" in his first public statement in over two weeks following the annexation of Crimea.
"As president who is with you in thought and soul, I ask every single sensible citizen of Ukraine not to let yourselves be used by the imposters! Demand a referendum on determining the status of every region in Ukraine," Yanukovych said in a statement quoted by Russia's ITAR-TASS news agency.
Russia entered the Crimean peninsula on the grounds that it has an "obligation" to protect the rights of Russian-speakers in the region.
The West has expressed concern that Russia could use the same pretext to expand its military presence in Ukraine, and ultimately take over eastern Ukraine.
Earlier this week, the Russian government opened a telephone hotline for Russians in Ukraine wishing to "complain about violations of their linguistic, cultural and religious" rights.
His call for a nationwide referendum comes as Ukraine's interim president Olexander Turchynov warned ultra-nationalist groups against further "destabilisation" after supporters of the Right Sector smashed windows and blocked the entrance to the parliament building on Thursday.
Speaking at a parliamentary session, Mr Turchynov said the Right Sector threatens to "destabilise the situation in Ukraine, in the very heart of Ukraine"- referring to the capital of Kiev- following the death of ultra-nationalist leader Oleksandr Muzychko.
Mr Muzychko, also known as Sashko Billy, was shot dead by police in a cafe in Rivne in western Ukraine. Supporters claim he was executed by the government despite playing a prominent role in the protests that led to the ousting of former president Viktor Yanukovych in February.
Adding to further turmoil, the Ukrainian government has warned the country is teetering "on the brink of economic and financial bankruptcy" and signalled more pain ahead after securing a $14-18 billion rescue package with the International Monetary Fund.
Mr Yatsenyuk said new structural reforms, including raising taxes and a freeze on minimum wage, will hit families hard but insisted the government had "no choice but the tell the truth".
In a statement, the IMF warned the country is facing "difficult" challenges following the "intense economic and political turbulence of recent months".
On Wednesday, Ukraine announced a radical 50 per cent increase in domestic gas prices starting 1 May in an effort to secure the IMF bailout, which made scrapping energy subsidies a key condition for the deal to move forward.