In a strong show of support, U.S. President Barack Obama has met Ukraine's Prime Minister Arseniy Yatsenyuk at the White House.
Obama told the Ukrainian leader that Washington will stand with Ukraine in ensuring that its territorial integrity is maintained.
"We have been very clear that we consider the Russian incursion into Crimea, outside of its bases, to be a violation of international law, of international agreements of which Russia is a signatory, and a violation of the territorial integrity and sovereignty of Ukraine," he said after the meeting on March 12.
Obama added that the West would be forced to "apply a cost" if Russian President Vladimir Putin did not change course in the dispute over Ukraine.He also said he is confident that the international community will stand firmly behind the Ukrainian government.
Yatsenyuk said his government is ready for talks, but will not abandon its fight to protect Ukraine's sovereignty:
"It's all about freedom," he said. "We fight for our freedom, we fight for our independence , we fight for our sovereignty and we will never surrender. "
The meeting in the White House comes just days ahead of a referendum in Ukraine's Crimea, now controlled by pro-Russian forces, on the region's joining Russia.
Obama said the United States would not recognize a referendum which he said would be illegal. But he said he hoped that last-ditch diplomatic efforts might lead to a "rethinking" of the planned vote.
Earlier on March 12, the leaders of the G7 group of advanced economies told Russia it risks facing international action unless it stops its moves toward the "annexation" of Crimea.
A statement from the seven nations released from the White House said a referendum on joining Russia, scheduled for this weekend, "would have no legal effect" and they won't recognize its results. It said Russia must "cease all efforts to change the status of Crimea."
The statement from the leaders of Canada, France, Germany, Italy, Japan, Britain and the United States -- along with the European Council and the European Commission -- said "further action, individually and collectively," will be taken against Russia if it proceeds with what it called the "annexation" of Crimea.
German Chancellor Angela Merkel, meanwhile, warned that the EU could impose targeted sanctions on Russian and Ukrainian officials next week if efforts at a diplomatic solution fail.
"Almost a week ago we said if that wasn't successful within a few days we'd have to consider a second stage of sanctions," she said. "Six days have gone by since then and we have to recognize, even though we'll continue our efforts to form a contact group, that we haven't made any progress."
Speaking after talks with Polish Prime Minister Donald Tusk in Warsaw on March 12, Merkel also said the political part of a key agreement between Ukraine and the European Union could be signed as early as next week.
She said the accord would be signed "probably at the next EU summit" scheduled for March 20-21.
Deposed Ukrainian President Viktor Yanukovych's sudden decision not to sign the agreement last November sparked mass protests that led to the toppling of his government and the subsequent Russian intervention in Crimea.
On March 12, U.S. Secretary of State John Kerry said he will travel to London to meet with his Russian counterpart Sergei Lavrov on March 14 in a last-ditch effort for a diplomatic solution.
In related news, a U.S. Senate panel has passed a bill authorizing $1 billion in loan guarantees to Ukraine's new government and allowing Washington to impose economic penalties on Russian officials responsible for the military intervention in Crimea.
The Senate Foreign Relations Committee approved the bill by a 14-3 vote, but the bill must still go to the full Senate and then the U.S. House of Representatives for approval before it could become law.
The Senate Committee bill stopped short of going after Russian banks or energy companies as some legislators proposed, giving John Kerry more leeway ahead of his talks with Sergei Lavrov later this week.
With reporting by Reuters, AP, and AFP