Armed men stand outside of Ukraine border post.
Russia said on Tuesday that it would retaliate if the United States imposed sanctions over Moscow's actions in Ukraine.
"We will have to respond," Foreign Ministry spokesman Alexander Lukashevich said in a statement. "As always in such situations, provoked by rash and irresponsible actions by Washington, we stress: This is not our choice."
US President Barack Obama said on Tuesday that Russian President Vladimir Putin's intervention in Ukraine is not a sign of Russian strength but rather a reflection of the deep concern Russia's neighbors have about Moscow's meddling.
In remarks to reporters, Obama ridiculed Putin's justification for any Russian military action in the Crimea region of southern Ukraine.
Putin has denied that Russian armed forces were directly engaged in the bloodless seizure of Crimea but said he has the right to send in military forces to protect Russian nationals. Obama has called Moscow's moves a violation of international law.
"President Putin seems to have a different set of lawyers making a different set of interpretations," Obama said. "But I don't think that's fooling anybody."
Obama said he held a meeting of the National Security Council on Tuesday morning, his second such session in two days about Ukraine.
Obama, who has drawn fire from Republicans for his handling of Ukraine, rejected suggestions that the Russian move was a clear strategic step.
"I actually think that this has not been a sign of strength but rather a reflection that countries near Russia have deep concerns and suspicions about this kind of meddling and if anything will push some countries further away from Russia," he said.
Obama said a US aid package is aimed in part at making sure Ukraine has elections and that legitimate elections should show the country is able to govern itself.
He urged Congress to back the aid package.
On Tuesday, Ukraine's telecommunications system has come under attack, with equipment installed in Russian-controlled Crimea used to interfere with the mobile phones of members of parliament, the head of Ukraine's SBU security service said.
Some Internet and telephone services were severed after Russian forces seized control of airfields and key installations in Ukraine's Crimea region on Friday, but now lawmakers were being targeted, Valentyn Nalivaichenko told a news briefing.
"I confirm that an...attack is under way on mobile phones of members of Ukrainian parliament for the second day in row," the security chief told a news briefing.
"At the entrance to (telecoms firm) Ukrtelecom in Crimea, illegally and in violation of all commercial contracts, was installed equipment that blocks my phone as well as the phones of other deputies, regardless of their political affiliation," he said.
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