WASHINGTON, March 15 -- The United States is closely watching the crisis in the Ukraine on the eve of a historic referendum that could see that country's autonomous region of Crimea break away and join Russia.

The U.S. and Russia have been at loggerheads during the last two weeks over Moscow's deployment of troops to the embattled Crimea region in what Russian President Vladimir Putin said was a bid to protect more than one million ethnic Russians in the area.

As a result, Washington has been ratcheting up pressure on Moscow, as the U.S. believes Sunday's vote violates the Ukraine's sovereignty and territorial integrity.

Earlier this week, the White House indicated that Moscow would be booted out of the Group of Eight (G8) industrialized nations over the controversy, and a bill is working through U.S. Congress to enact financial sanctions on Russia.

U.S. President Barack Obama said Friday he still hoped that "there is a diplomatic solution to be found," but that "the United States and Europe stand united, not only in its message about the Ukrainian sovereignty but also that there will be consequences if, in fact, that sovereignty continues to be violated."

For his part, Putin last week dismissed assumptions that Moscow is considering Crimea's accession to Russia, adding that only the Crimean people themselves may determine their future.

Russian Foreign Minister Sergei Lavrov echoed those thoughts on Friday after holding several hours of talks in London with U.S. Secretary of State John Kerry, saying that Russia has no plans to invade the Crimea.

Still, the U.S. has in recent weeks threatened Russia with sanctions, although it remains unknown whether that would force a Russian withdrawal from the region.

Russia currently supplies around a quarter of Europe's oil and natural gas, not to mention 40 percent of economic powerhouse Germany's gas. Moscow has a deep trade relationship with Eurozone nations, which some experts said could make Europe hesitate to impose sanctions.

Russia sends nearly 300 billion U.S. dollars worth of exports to EU countries, which account for nearly half of Russia's total exports, whereas Russia is only the 20th largest trading partner for the U.S.

Some experts said a "yes" vote in the Crimea referendum on Sunday to break away from the Ukraine could cause U.S.-Russia relations to unravel.

"The important thing (to watch) will be Russia's reaction in the event of a 'yes' vote. If (Russia) decides to respond by formally annexing Crimea, it will be crossing a line that puts it on a collision course with the U.S. and the West more generally," David Clark, chairman of the Russia Foundation, told Xinhua.

Meanwhile, U.S. critics blasted Obama for what they said was putting the U.S. policy on Russia on the back burner, and for causing Moscow to view Washington as weak.

Clark said Putin reflects two-sided Russian nationalist view of the U.S. and the West. On the one hand, Russia sees them as aggressive and domineering, but at the same time views them as weak and decadent.

"The fall of (Ukraine's President Viktor) Yanukovych's government was interpreted as an act of U.S. assertiveness to which Russia had to respond. His seizure of Crimea was an attempt to recover lost ground and test Western responses," Clark said.