Russia's leaders are refusing all negotiations with their Ukrainian counterparts, Ukraine's acting President Oleksandr Turchynov has said.
He told AFP news 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," Mr 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.
Mr Turchynov earlier on Tuesday called for the creation of a national guard and provide support to troops.
He said that the force would include 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 Mr 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 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.
In other developments:
- An adviser to Ukrainian presidential hopeful Vitali Klitschko has warned that if Crimea is annexed by Russia, European security will "go up in flames"
- The European Commission on Tuesday offered Ukraine trade incentives worth nearly 500m euros ($694m; £417m) to stabilise the country's crisis-hit economy
- Ukrainian troops remain blockaded in their bases by armed men across Crimea
- Intense diplomatic efforts to settle the crisis diplomatically are continuing, with US Secretary of State John Kerry and his Russian counterpart Sergei Lavrov speaking by telephone on Tuesday
- At a news conference in Russia on Tuesday, ousted President Yanukovych described the new Ukrainian authorities as a "gang of fascists" and says presidential elections set for 25 May are "illegal"
- A number of flights from Crimea's main airport in Simferopol have been cancelled amid reports that pro-Moscow militia have taken over air traffic control