Russian President Vladimir Putin on Tuesday night made it official that Crimea would join Russia, in defiance of the West.
Addressing a joint session of parliament, Putin said Crimea was of vital importance to Russia, underlining its historical and cultural links.
He added that Moscow's actions were responding to a call for help.
"Residents of Crimea and Sevastopol called on Russia to protect their rights and lives, to prevent what happened and is still happening in Kiev, Donetsk, Karkiv and other Ukrainian cities. Naturally, we could not dismiss such request, we could not leave Crimea and its residents in peril. It would have been a betrayal."
Putin then signed a treaty with Crimean Prime Minister Sergei Aksyonov for the Ukrainian Black Sea peninsula to be become part of Russia.
Following the signing ceremony, the Kremlin announced that it now considers Crimea Russian territory, despite strong objections from the international community.
Western powers continue to push for greater sanctions on Moscow over its intervention in the region.
Earlier Tuesday, the European Union and the U.S. imposed sanctions and asset freezes against Russian and Ukrainian officials.
"we are imposing sanctions on specific individuals responsible for undermining the sovereignty, territorial integrity and government of Ukraine."
The sanctions, which include travel bans were in retaliation for Sunday's referendum vote in Crimea which was deemed illegal by Western leaders.
Meanwhile, Kiev has approved the mobilization of up to 40-thousand troops to counter "Russian interference" in the country.
Paul Yi, Arirang News.
And within the past couple of hours, Ukraine's military has confirmed that a serviceman has been killed in an attack on a base in Crimea, the first such death since pro-Russia forces took control last month.
It is not yet clear who staged the assualt, but Ukraine has now authorized its troops to fire in self defense.