The White House warned Russia to keep its troops out of Ukraine amid fears that Moscow may step in with military force following the overthrow of the president, its ally.
Tensions also mounted in Crimea, in the south-east of Ukraine, where pro-Russian politicians are organising rallies and forming protest units, demanding autonomy from the capital Kiev.
The region is now seen as a potential flashpoint because of its deep strategic significance to Moscow.
US President Barack Obama's national security adviser said it would be a "grave mistake" for Russian President Vladimir Putin to send soldiers into Ukraine to restore a friendly government after the upheaval.
Susan Rice said nobody would benefit if Ukraine were to split apart.
"It's in nobody's interest to see violence return and the situation escalate," Ms Rice said.
Her warning to the Kremlin followed concerns over renewed tumult in Ukraine if eastern regions of the vast country side with Russia against the overthrow of Viktor Yanukovych.
Ukraine is deeply divided between its eastern regions, which are largely pro-Russian, and western areas that widely detest Mr Yanukovych.
The Crimean port of Sevastopol may be part of Ukraine, but it is the Russian tricolor that flutters proudly above the port's barrack blocks and warships.
The city's cobbled streets are full of Russian sailors, often raucously drunk, while the harbour shelters ranks of sleek Russian vessels that dwarf their Ukrainian neighbours.
Under a deeply politically divisive leasing deal, the deep-water port is home to a huge naval base and the Russian Black Sea Fleet, providing Russia's military with easy access to the Mediterranean.
However, many Russians believe that it is only a twist of fate that means the peninsula is not part of their country anyway – and turmoil in Ukraine could prove a perfect opportunity to reassert their claim.
Sevastopol has been a proud part of Russian imperial might since the 18th Century, but in 1954 it was transferred to Ukrainian control under Nikita Khrushchev, an ethnic Ukrainian.
When Ukraine broke away from the Soviet Union in 1991, it took Crimea with it. Moscow has since had to lease the strategically critical naval base.
Lilit Gevorgyan, a senior Russia analyst at IHS Jane's Insight, said: "There are many Russians who believe it was Khrushchev, who was an ethnic Ukranian, who decided to give it to Ukraine, and still believe it is unfair.
"Strategically, symbolically and historically, it is important for the Russians. If there's turmoil and real talk of the break-up of Ukraine, the Russians will be interested in securing this part."
In a recent opinion poll, 56pc of Russians said they viewed Crimea as a Russian territory, a far higher proportion than felt a claim on Chechnya.
However, Ms Gevorgyan said the Crimean population was extremely diverse, and it may prove difficult to manipulate by Russian nationalists.
In particular, the region's significant population of Muslim Tartars, who suffered persecution and mass deportation under Stalin, have little desire to join Russia.
"It's a patchwork of different identities and I am not sure it will be easy to manipulate," she said.
"It has never been either truly Russian or truly Ukrainian," she added.
Ms Rice said it would be a mistake for Mr Putin to view the tumult as a Cold War battle between the East and West. (© Daily Telegraph, London)