Ukraine's new Western-backed prime minister said Tuesday that the ex-Soviet country had no plans to join NATO following last month's fall of a pro-Kremlin regime.

Prime Minister Arseniy Yatsenyuk delivered a special address to the nation designed to ease tensions between Ukrainian nationalists who spearheaded three months of protests against the pro-Moscow authorities and Russian speakers who view the new pro-European government in Kiev with mistrust.

President Vladimir Putin on Tuesday signed a treaty making Crimea part of Russia, in a historic redrawing of Russia's borders after he declared that the Black Sea region has always been "in the hearts" of his countrymen.

In a fast-moving sequence of events following Crimea's controversial secession referendum on Sunday, the Kremlin said Crimea was now considered part of Russia and no longer Ukrainian territory, shrugging off strong international objections.

Japan on Tuesday said it would slap sanctions on Russia over its "deplorable" move recognizing a Crimean vote to break away from Ukraine.

Russian President Vladimir Putin on Monday signed a decree recognizing Crimea as an independent state following a weekend referendum to secede from Ukraine and join Russia in a move that has fanned the worst East-West crisis since the Cold War.

The world should welcome the prospect of Crimea becoming part of Russia as it rectifies a historic mistake from the Soviet era, the Soviet Union's last leader Mikhail Gorbachev said Monday.

Gorbachev said that Crimea had only ended up in the territory of post-Soviet Ukraine because it had been transferred from Russia by Soviet leader Nikita Khrushchev when both countries were part of the USSR.

The United States on Monday imposed financial sanctions on seven top Russian government officials and lawmakers to punish Russia's incursion into Crimea.

In a new executive order, President Barack Obama also imposed sanctions against former Ukrainian President Viktor Yanukovych and one of his top advisors and two top "separatist" leaders in Crimea.

Russian aggression in separatist Crimea was prompted by "panic" in Moscow over a democratic and European Ukraine, Ukraine's interim president Oleksandr Turchynov said Monday.

Moscow was "in a panic over Ukraine's example where the people overcame a totalitarian regime. The Kremlin is most afraid of a democratic, European and prosperous Ukraine... that's the reason for its aggression," he said in a speech to the nation.

U.N. Secretary General Ban Ki-moon said Monday he was "deeply disappointed and concerned" about the impact of Sunday's vote in Crimea to break away from Ukraine.

"The secretary general once again urges all parties to refrain from violence and to commit themselves to de-escalation and inclusive national dialogue in the pursuit of a political and diplomatic solution," his spokesman said.

Crimea's regional assembly on Monday declared independence from Ukraine and applied to join Russia, saying all Ukrainian state property on the peninsula would be nationalized.

Later on Monday, Russian President Vladimir Putin signed a decree formally recognizing Crimea as an independent state following its vote to secede from Ukraine and join the Russian Federation.

Japan on Monday said it would "strongly urge" Russia not to annex Crimea, joining a chorus of criticism over a weekend referendum by the region of Ukraine as Washington hinted at fresh sanctions against Moscow.

With more than half the ballots counted, preliminary results showed 95.5 percent of voters were in favor of leaving Ukraine and rejoining former political master Russia.

An overwhelming 95.5 percent of Crimeans voted Sunday to become part of Russia in a referendum deemed illegal by the new authorities in Ukraine and most of the international community, exit polls showed.

With 50 percent of ballots counted, referendum commission chairman Mykhaylo Malyshev said 3.5 percent had voted to remain in Ukraine with wider autonomous powers and 1.0 percent were "spoiled ballots."