The Ukrainian government has authorised its soldiers in Crimea to use weapons to protect their lives, reversing previous orders that they should avoid using arms against attack.
A Ukrainian soldier was killed earlier on Tuesday when a base still held by Kiev came under attack in Simferopol.
It's the first death in Crimea from a military clash since Russia seized overall control of the region three weeks ago.
The government in Kiev says masked attackers wearing Russian military uniforms detained all the troops at the base and confiscated their weapons, IDs and money.
The incident occurred shortly after Crimea's leaders and Russia's President Vladimir Putin signed a treaty making the region part of the Russian Federation on Tuesday.
Mr Putin told parliament after the signing that Crimea, which was taken over by pro-Russian forces in February, had "always been part of Russia", the BBC reports.
Deputy Ukrainian minister of foreign affairs Danielo Lubkivsky says Mr Putin's actions are "unbelievable".
Kiev says it will never accept the treaty and the United States has called a G7-European Union crisis meeting next week in The Hague.
US Vice-President Joe Biden called Russia's involvement in Crimea "a brazen military incursion" and its annexation of the territory was "nothing more than a land grab" by Moscow.
Germany and France quickly condemned the Russia-Crimea treaty.
British Prime Minister David Cameron said: "It is completely unacceptable for Russia to use force to change borders on the basis of a sham referendum held at the barrel of a Russian gun."
After the Treaty was signed, Mr Putin appeared before crowds in Moscow's Red Square, telling them: "Crimea and Sevastopol are returning to... their home shores, to their home port, to Russia!" He shouted "Glory to Russia" as the crowds chanted "Putin!"
The Ukrainian crisis began in November last year after pro-Moscow President Viktor Yanukovych abandoned an European Union deal in favour of stronger ties with Russia.
The United States and its G7 allies will gather next week at The Hague to consider further response to the situation.
The meeting will take place on the margins of a nuclear security summit that American president, Barack Obama, plans to attend.
The G7 nations are the United States, Germany, Britain, Canada, France, Japan and Italy. The European Union has also been invited to the talks.
The G7 leaders have already have suspended preparations for a G8 summit, which includes Russia, that had been scheduled to take place in the Russian city of Sochi in June.