Russian President Vladimir Putin and Crimean leaders have signed treaties making Ukraine's Crimea and the city of Sevastopol part of the Russian Federation.
Addressing a special joint session of the Russian parliament ahead of the signing ceremony on March 18, Putin said to a standing ovation that "in the hearts and minds of people, Crimea has always been and remains an inseparable part of Russia."
He said Crimea's secessionist referendum on March 16 was democratic and in full accordance with international law. He said results of the referendum -- in which Crimea's secessionist authorities said 97 percent of voters backed joining Russia -- were "fully convincing."
He said Russia "could not ignore the pleas [for help]. We could not leave Crimea and its people in trouble. It would have been nothing short of betrayal."
WATCH: Putin makes the announcement to the two houses of parliament.
The treaties were signed on March 18 by Putin; regional parliament speaker Vladimir Konstantinov; secessionist Prime Minister Sergei Aksyonov; and Sevastopol's self-declared mayor, Aleksei Chaly.
The United States and the European Union have rejected Crimea's referendum as illegal and a violation of the Ukrainian Constitution and imposed sanctions on Russians and Ukrainians linked to the secessionist vote.
Dismissing Western criticism as a manifestation of double standards, Putin cited Kosovo's 2008 unilateral declaration of independence from Serbia, which has been recognized by Western states, although not by Russia.
Putin also said the rights of all nationalities in Crimea would be respected.
"We treat all nationalities living in Crimea with respect. It is their common home, their little motherland," he said. "Thus it would be right -- and I know that people in Crimea support this idea -- to introduce three official languages in Crimea: Russian, Ukrainian, and Crimean Tatar."
He said Russia did not want -- and did not "need" -- the partition of Ukraine. But he denounced Ukraine's new leadership for allegedly opening the doors to "nationalists" and "neo-Nazis" and said Russia will always defend the interests of Russians and Russian-speakers there.
"The main perpetrators of the [Ukrainian] coup were nationalists, neo-Nazis, Russophobes, and anti-Semites," Putin said. "These are exactly the people who are still up to this day in control of life in Ukraine."
He denied Western accusations that Russian forces had invaded Crimea, saying Moscow had sent troops in the region in line with a treaty with Ukraine. The treaty allows Russia to have up to 25,000 soldiers at its Black Sea Fleet base in Crimea.
Sevastopol, where the Black Sea Fleet is based, has a special autonomous status in Ukraine, necessitating two separate treaties.
He accused the West of reverting to Cold War containment and said Western states had crossed a "red line" over Ukraine.
"In their practical policies, our Western partners -- spearheaded by the United States of America -- prefer the rule of the strong to international law," Putin said. "They came to believe in their exclusivity, in being the chosen ones. They feel they are allowed to rule the fate of the world and that they are the only ones to always be right."
He thanked China for what he called its support. Beijing abstained on a UN Security Council resolution denouncing Russia's actions on Crimea, which Moscow had to veto on its own.
Lawmakers frequently interrupted Putin's speech with loud applause. Parliament is expected to quickly endorse the treaties on Crimea's annexation.
In a first immediate response, Ukraine's Foreign Ministry rejected Crimea's annexation by Russia, as well as its self-declared independence. A senior Foreign Ministry official said that "what happened today in Moscow has nothing in common with democracy, law, or common sense."
NATO Condemns 'Land Grab'
NATO member states have condemned Russia's annexation of Ukraine's Crimea as a "land grab" in violation of international law and warned that Moscow faces the prospect of more Western sanctions.
Speaking on a visit to NATO ally Poland, U.S. Vice President Joe Biden condemned Russia's annexation of Ukraine's Crimea as "nothing but a land grab." He said Russia's international isolation will only increase and Moscow will face additional U.S. and EU sanctions if it continues with moves to take over Crimea.
Biden also said the United States remained strongly committed to the collective defense of all NATO members and affirmed Washington's commitment to establishing a missile-defense system in Poland by 2018. Polish Prime Minister Donald Tusk, who appeared with Biden, said the international community cannot accept Russia's annexation of Crimea.
In London, Foreign Secretary William Hague said Britain had suspended all military cooperation with Russia following Moscow's annexation of Crimea.
In a statement to Parliament on March 18, Hague said Putin had chosen the path to Russia's isolation by his actions in Crimea. He said Putin was "denying the citizens of his own country, and of Crimea, partnership with the international community and full membership of a range of international organizations."
He also warned of the danger that a provocation elsewhere in Ukraine could be used as a pretext for a further military escalation.
German Chancellor Angela Merkel said the referendum in Crimea, the declaration of independence by secessionist Crimean lawmakers, and Russia’s annexation of the territory were all against international law. French President Francois Hollande has urged a "strong" EU response.
With reporting by Reuters, AFP, AP, and Interfax