Russia's leaders are refusing all negotiations with their Ukrainian counterparts, Ukraine's acting President Oleksandr Turchynov has said.
Turchynov told the AFP agency that Ukraine would not intervene militarily in Crimea, even though a secession referendum there was a "sham".
Meanwhile interim Prime Minister Arseniy Yatsenyuk is travelling to the US to meet President Barack Obama.
On Thursday he is due to address the UN Security Council in New York.
"We cannot launch a military operation in Crimea, as we would expose the eastern border [close to Russia] and Ukraine would not be protected," Turchynov told AFP.
He said that Sunday's referendum in Crimea - in which people on the peninsula will decide whether or not to become part of Russia - was "a provocation" that would be boycotted by most people.
"The Russian forces don't intend to hold a referendum, they're just going to falsify the results," he said.
The president said that at the same time the Russian government was refusing to enter into any dialogue with Ukraine.
"Unfortunately, for now Russia is rejecting a diplomatic solution to the conflict," he told AFP.
"They are refusing all contact at foreign ministry and top government level."
Turchynov earlier on Tuesday called for the creation of a national guard to defend the country and provide support to just 6,000 troops who are combat-ready.
He said that the force would comprise volunteers with military experience who would be on guard against external and internal aggression.
The president was speaking as Moscow announced more military exercises involving 4,000 paratroopers - apparently the biggest such exercise in 20 years.
Meanwhile Prime Minister Arseniy Yatsenyuk flies to the US on Wednesday and is scheduled to meet President Obama on the same day in the White House.
Correspondents say that Yatsenyuk is likely to discuss the details of a $35bn (£21bn) aid package he says that Ukraine's teetering economy needs to stay afloat over the next two years.
The BBC's Mark Mardell in Washington says that President Obama is likely to use the meeting to again call on the Russians to return their troops to base, support elections in May and enter into direct negotiations with Ukraine's government.