Ukraine has put its armed forces on full combat alert and warned Moscow that military intervention will lead to war after the upper house of Russia's parliament gave President Vladimir Putin the authority to invade.
After a more than three-hour meeting with security and defence chiefs, Oleksander Turchinov, Ukraine's interim president, said there was no justification for what he called Russian aggression against his country.
Turchinov said that authorities had boosted security around the country’s nuclear facilities, airports and other "strategic facilities".
Standing beside Turchinov, Arseny Yatseniuk, Ukraine's prime minister, said he had urged Russia to return its troops to base in the Crimea region during a phone call with Dmitry Medvedev, Russia's prime minister, and called for talks.
"I am convinced that Russia will not launch an intervention as this would mean war and the end of all relations between the two countries," he said.
Putin and US President Barack Obama held a 90-minute telephone call on Saturday, in which the Russian president told his counterpart that Russia reserves the right to protect its interests and those of Russian speakers if there is violence in east Ukraine or Crimea, Russia's RIA Novosti news agency reported.
The Federation Council, the upper house of the Russian parliament, voted overwhelmingly on Saturday to back a proposal by Putin to use "the armed forces of the Russian Federation on the territory of Ukraine until the normalisation of the socio-political situation in that country".
The developments come after Sergiy Aksyonov, the pro-Russian prime minister of the Crimea region, appealed to Putin for help to ensure peace in the semi-autonomous region where unidentified gunmen believed to be Russian are guarding state buildings, including the local parliament.
The region has a majority of ethnic Russians and its administration is defying the new Kiev government that ousted Moscow-backed Ukrainian President Viktor Yanukovich.
The 15-member UN Security Council held emergency talks in New York on Saturday to discuss the crisis.
Samantha Power, the US ambassador to the UN, said that Russia's parliamentary approval was "as dangerous as it is destabilising" and accused Moscow of acting without legal basis.
"The message is pull back your forces. Let us engage in political dialogue, engage with the Ukrainian government which is reaching out to you for that dialogue."
In a statement, US Secretary of State John Kerry said: "The United States condemns the Russian Federation's invasion and occupation of Ukrainian territory, and its violation of Ukrainian sovereignty and territorial integrity ... This action is a threat to the peace and security of Ukraine, and the wider region."
Washington has said it will suspend participation in preparatory meeting for the G8 summit in Sochi, Russia, planned for this summer, the Reuters news agency reported.
In a statement issued after an emergency cabinet meeting on Saturday, Canada's prime minister said he had also suspended his country's participation.
Stephen Harper said: "Canada has suspended its engagement in preparations for the G-8 Summit, currently planned for Sochi, and the Canadian ambassador in Moscow is being recalled for consultations."
Anders Fogh Rasmussen, NATO's secretary general, said NATO ambassadors will meet in Brussels on Sunday to discuss the situation in Ukraine.
Rasmussen tweeted on Saturday: "North Atlantic Council will meet tomorrow followed by NATO-Ukraine Commission."
A Kremlin statement quoted by RIA said that Putin has not yet taken a decision on sending troops to Ukraine.
European Union foreign policy chief Catherine Ashton urged Russia to refrain from deploying its armed forces in Ukraine and to abide by international law.
The United Nations Secretary-General Ban Ki-moon called for the immediate restoration of calm in Ukraine as the Security Council held emergency talks on the escalating crisis.
Al Jazeera's Tim Friend, reporting from Kiev, said: "This [the approval of Putin's request] is formalisation of what they regard as already happening - the Russian annexation of Crimea.
"They want the UN to be involved and calm the situation in the region."
Friend said the tense atmosphere in Crimea is spreading to other parts of the country such as the eastern city of Donetsk, where EU and Russia supporters have clashed.
"This is a very worrying situation," he said.
Russia, denying accusations of staging an aggression against its neighbour country, says any military movement in Crimea is part of the agreement in question that was previously made.
Moscow is also refusing to hold talks with Kiev under the auspices of that agreement, which guarantees Ukraine's territorial integrity, Interfax news agency quoted Ukrainian Foreign Minister Andrij Deshchitsya as saying on Saturday.
Ukraine's new government, which came to power after the ousting of Viktor Yanukovich, has called for fresh presidential elections on May 25, a move opposed by the administration in Crimea.
The interim government has issued an arrest warrant for Yanukovich, accusing him of being responsible for the deaths of at least 70 protesters on February 21 protests in Kiev.