Crimea's regional assembly has officially applied to become part of Russia, a day after a referendum in southern Ukraine overwhelmingly supported joining the Russian Federation. Crimea's parliament approved the measure Monday.
The United States and its European allies are expected to announce sanctions Monday against Russia.
President Barack Obama told his Russian counterpart Vladimir Putin Sunday that Washington and its "European partners are prepared to impose additional costs" on Moscow for backing the secession referendum in Ukraine's Crimean peninsula.
A White House statement called Sunday's referendum illegal and said it violates Ukraine's constitution. It also said the vote will "never be recognized by the United States and the international community."
Crimea's election chief announced Monday that nearly 97 percent of the voters cast ballots supporting secession and a move to join Russia. However, those opposed to the move had been advised to boycott the referendum.
In Kyiv, Ukraine's interim Prime Minister Arseniy Yatsenyuk - speaking at an emergency cabinet meeting - called the Moscow-backed Crimea vote "a circus spectacle" directed at gunpoint by Russia.
An earlier White House statement said no decision should be made about the future of Ukraine without the Ukrainian national government.
It also said the presidential elections planned for May 25 will provide a legitimate opportunity for all Ukrainians to make their voices heard on the future of their country.
Secretary of State John Kerry and Russian Foreign Minister Sergei Lavrov on Sunday agreed to push for Ukrainian constitutional reforms for power sharing and decentralization as a solution to the crisis.
The Duma set to act
Meanwhile, Russia's lower house of parliament will pass legislation allowing Ukraine's Crimea region to join Russia “in the very near future”, Interfax news agency quoted the chamber's deputy speaker as saying on Monday.
Russian President Vladimir Putin has said he will respect the will of the people in Crimea, ignoring Western leaders who say the referendum was illegal because Russian forces have seized the southern region.
“The results of the referendum in Crimea clearly showed that residents of Crimea see their future only as part of Russia,” the deputy speaker of the State Duma, Sergei Neverov, was quoted as saying.
Duma officials claim the Black Sea peninsula can become a member of the Russian Federation under current legislation, specifically under a law “On the procedure for the adoption into the Russian Federation and education of new subjects of the Russian Federation” that was passed in 2001, Interfax said.
First, Crimea's appeal to join Russia will be sent to Putin. If approved, Putin will then pass it to the upper and lower houses of parliament, which will work on a treaty to be signed between Russia and the new state.
Under the treaty, a transitional period could be set for the new subject to be integrated into Russia's economic, financial, credit and legal systems.
Following its signing, Russia's constitutional court should then verify the treaty. It should then be voted on by both houses of parliament, the Duma and the Federation Council.
“I do not think there will be any delays in considering these questions in either the State Duma or Federation Council. We are ready to pass all the required legal decisions as quickly as possible,” the Federation Council's deputy speaker, Ilyas Umakhanov, told Rossiya-24 television.
Situation on the ground
A man fills out registration papers before casting his vote in a mobile ballot box during voting in a referendum in the village of Pionerskoye, near Simferopol, Crimea, Ukraine, March 16, 2014.
Election officials arrive with a mobile ballot box at a house during voting in a referendum in the village of Pionerskoye, Crimea, Ukraine, March 16, 2014.
A woman holds a Russian flag as she casts her ballot during the referendum on the status of Ukraine's Crimea region at a polling station in Bakhchisaray, Crimea, Ukraine, March 16, 2014.
Crimean Prime Minister Sergei Aksyonov (C) leaves after casting his ballot during voting in a referendum at a polling station in Simferopol, Crimea, Ukraine, March 16, 2014.
A woman holds her dog inside a polling station during the referendum on the status of Ukraine's Crimea region in Sevastopol, Crimea, Ukraine, March 16, 2014.
Pro-Russian demonstrators hold large copies of United Nations documents on the right to self-determination outside the regional parliament building in Simferopol, Crimea, Ukraine, March 16, 2014.
A woman casts her ballot during the referendum on the status of Ukraine's Crimea region at a polling station in Bakhchisaray, Crimea, Ukraine, March 16, 2014.
A woman casts her ballot during voting in a referendum at a polling station in Simferopol, Crimea, Ukraine, March 16, 2014.
An armed man, believed to be a Russian serviceman, stands guard outside a Ukrainian military base in Perevalnoye, near Simferopol March 16, 2014.
People shout slogans during a pro-Russian rally at a central square in Donetsk, eastern Ukraine, March 15, 2014.
In Kyiv, Ukraine's acting defense minister told reporters that both Ukraine and Russia have agreed on a truce in Crimea until March 21.
Sunday's vote came a day after Russian forces seized a natural gas facility just outside Crimean territory. The Ukrainian Foreign Ministry called the move "a military invasion by Russia."
Ukraine provides the peninsula with all of its water and energy needs, and some analysts say the seizure may be aimed at ensuring the peninsula's energy requirements are met in the event Kyiv were to cut off supplies.
Crimea is a primarily ethnic-Russian region within Ukraine. Moscow says it has the right to protect the interests of ethnic Russians in Crimea.
Ukraine's Foreign Ministry said Saturday Russian forces have seized the Ukrainian village of Strilkove, near the Crimean border.
There are no reports of shots being fired, but the ministry called the takeover an "invasion" and demands that Russian soldiers leave. Ukrainian border guards say the Russians are guarding a gas pumping station in the town.
Reactions in Kyiv
Thousands of Ukrainians gathered in central Kyiv Sunday to voice opposition to the referendum and what the perceive as Moscow's moves to divide the Ukraine.
But the mood was somber as many Ukrainians feel helpless against Russia's might and military superiority, many fearing a further escalation of tensions.
Irina, a restaurant manager who only gave her first name, said Crimea's fate likely was already decided in Moscow.
She said none of this was right. This could have been done in a nice way, in an honest way, she said. This could have been done in a constitutionally correct way. And it seems to me, she said, everyone would have agreed to that.
Moscow claims it is protecting ethnic Russians from persecution by Ukrainian “extremists” who it says illegitimately came to power after months of anti-government protests.
Another Kyiv resident, Ira, who also only gave her first name, said she had nothing against Russians.
She said she loves and respects the Russian people as much as Ukrainians, but not their government. She expressed hope that everything ends well, everyone becomes united, and that Crimea remains with Ukraine.
VOA's Daniel Schearf contribited to this report from Kyiv. Some of this report was contributed by Reuters.