Armed men smashed a Russian military truck through the gates of a Ukrainian missile defense post in the Crimean peninsula.
The men negotiated with the base's commanders as soldiers sheltered in their barracks Friday. No shots have been reported.
Earlier Friday, Ukraine's Interim President Oleksandr Turchynov signed a decree canceling a planned referendum on Crimea joining Russia.
A day earlier,Crimea's Moscow-backed legislature voted for the peninsula to become part of Russia and scheduled a referendum on the issue for March 16.
Ukraine's interim prime minister says that "no one in the civilized world" will recognize the referendum's results.
Arseniy Yatsenyuk says he wants to "warn separatists" and others he describes as "traitors of the Ukrainian state" that their decisions are "unlawful" and "unconstitutional." U.S. and European leaders also called the referendum illegal.
Crimean officials fired back Friday, saying the vote will go forward.
The speaker of Russia's upper house of parliament said Friday Russian lawmakers will support Crimea's decision if the Ukrainian region decides to join Russia. Tens of thousands of people turned out for a rally in the Russian capital to show solidarity with Crimea's pro-Russian authorities.
Ukraine and Russia have been locked in a tense standoff since Russian forces entered the Crimean peninsula a week ago.
Also Friday, Russian President Vladimir Putin's spokesman said he hoped that Russia and the West do not return to a period of tensions like the Cold War.
Russian news agencies quoted Dmitry Peskov as saying that despite "profound disagreements" between the two sides, "the hope remains that as a result of dialogue it will be possible to find some common ground."
Still, Peskov dismissed the idea that Western countries could mediate talks between Russia and Ukraine.
U.S. President Barack Obama spoke by phone Thursday with Russian President Vladimir Putin about the Ukraine crisis, but the two leaders found little common ground.
The White House says Mr. Obama told Mr. Putin the presence of Russian forces in Crimea is a violation of Ukraine's sovereignty. The Kremlin says Mr. Putin denounced Ukraine's new government as "illegitimate" and said Russia cannot ignore calls for help from Ukraine's Russia-leaning east and south.
Also Thursday, Mr. Obama authorized sanctions, including visa restrictions, against those found to have violated Ukraine's territorial integrity. The EU also took measures against Russia, suspending talks on visas and a new economic agreement.
Russia's Foreign Ministry on Friday called the EU's position "extremely unconstructive," adding that Russia "will not accept the language of sanctions and threats" and promising retaliation if the EU imposes sanctions.
Ukrainian Prime Minister Yatsenyuk said Friday his government is "prepared to rebuild relations with Russia." But he said Russia must withdraw its troops, fulfill its agreements with Ukraine and stop supporting separatists in Crimea.