Crimea referendum showed support for a split from Ukraine with election officials saying that 96.6% of voters backed joining Russia.
The government in Kiev described the vote as a “circus performance” and said it would not recognize the result.
The EU and US say the referendum was illegal, but Russia says it was consistent with international law.
President Barack Obama has warned Moscow that Washington is also ready to impose “costs” over its actions in Ukraine.
As polls closed on Sunday night, the White House said the international community “will not recognize the results of a poll administered under threats of violence”, describing Russia’s actions as “dangerous and destabilizing”.
The new authorities in Kiev say Russian troops moved in to Crimea after Ukraine’s pro-Moscow president, Viktor Yanukovych, was ousted following months of street protests.
Crimean leaders are expected to formally apply to the Kremlin on Monday to join Russia.
The Russian parliament had been expected to wait until Friday to begin debating the relevant legislation. It is now believed the process of absorbing Crimea could take place under existing laws.
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