Ukraine will respond with military action if Russia attempts to annex the country's mainly Russian-speaking eastern regions, interim prime minister Arseniy Yatsenyuk has said.
"I want to officially warn Russia: we will respond firmly, including through military means, against any attempt to seize Ukraine, to cross borders, or annex eastern or other regions by Russian troops," Yatsenyuk was quoted yesterday as saying in Brussels on the government website.
Yatsenyuk also appealed to the West to "respond appropriately" as Moscow moves to attach Ukraine's Crimea Peninsula to the Russian Federation.
"Russia has violated international law and undermined the nuclear non-proliferation regime," the premier said.
"Russia has carried out an armed robbery against an independent neighbouring country."
Under a milestone agreement in 1994, Russia agreed to respect Ukraine's territorial integrity while Kiev renounced its Soviet-era nuclear arms.
"Everyone should understand that there is a price to pay for stability in the world," said Yatsenyuk.
"There are two means: either with victims (of a conflict) or with euros and dollars," he said in reference to economic sanctions.
"It is better to sacrifice euros and dollars than to cry over thousands of deaths in a bloody war.
"I hope that our European partners understand that. Afterwards it will be too late to use other types of sanctions," he added.
In an address to the German Parliament in Berlin, Chancellor Angela Merkel said the EU was readying further sanctions and that the G-8 forum of leading economies had been suspended indefinitely.
Russia holds the presidency of the G-8 and President Vladimir Putin was due to host his counterparts, including President Barack Obama, at a G-8 summit in Sochi in June.
"So long as there aren't the political circumstances, like now, for an important format like the G-8, then there is no G-8," Merkel said. "Neither the summit, nor the format."
The US and the EU have slapped sanctions on individuals involved in what they say was Crimea's unlawful referendum over joining Russia. Moscow formally annexed Crimea earlier this week in the wake of the poll.
The Black Sea peninsula had been part of Russia for centuries until 1954 when Soviet leader Nikita Khrushchev transferred it to Ukraine, and many residents were happy about rejoining Russia.
Russian forces effectively took control of Crimea some two weeks ago in the wake of the ouster of Ukraine's pro-Russia president, Viktor Yanukovych, after months of protests and sporadic violence.