Simferopol, Ukraine (CNN) -- Diplomats scrambled Saturday to respond to the unanimous vote by Russia's upper house of Parliament to approve sending military forces into Ukraine.
The vote followed a request from President Vladimir Putin for approval to send troops into Crimea to normalize the political situation there.
Putin cited the "extraordinary situation in Ukraine" in making his request, adding that the lives of Russian citizens and military personnel based in the southern Crimea region had been threatened.
Late in the evening, Ukrainian acting Prime Minister Arseniy Yatsenyuk said in a televised address in Kiev that he had been told by Russian Prime Minister Dmitry Medvedev that no decision had been made on whether Russian military forces would indeed enter his country.
Yatsenyuk said his country was ready to mobilize its forces to protect strategic locations, including nuclear power plants.
The Ukrainian military is at the highest state of military readiness, Defense Minister Igor Tenyukh said. He credited negotiations during the day between the Ukrainian and Russian naval chiefs with easing tensions and said more negotiations were planned for Sunday.
Ukraine's new government condemned the move by Russia's upper house.
"We perceive Russia's actions as direct aggression towards the sovereignty of Ukraine," said acting President Oleksandr Turchynov on the Twitter account of his Fatherland party.
Ukrainian opposition leader Vitali Klitschko said that he planned to ask Turchynov to call for parliament to meet in emergency session to vote to invalidate the Black Sea Fleet Naval Base agreement with Russia.
Crimea's pro-Russian leader asked Putin for help
The Russian upper house vote came on the day that the newly installed, pro-Russian leader of Crimea, Sergey Aksyonov, asked Putin for help in maintaining peace on the Black Sea peninsula -- where Russia's fleet is based at Sevastopol.
Security forces "are unable to efficiently control the situation in the republic," he said in comments broadcast on Russian state channel Russia 24. Aksyonov was installed as the region's premier after armed men took over the Crimean Parliament building on Thursday.
Aksyonov said Saturday that a referendum on greater Crimean autonomy, originally set for May 25, would be moved to March 30.
Ukrainian government officials suspect Moscow of fomenting separatist tensions in the autonomous, majority-Russian region -- and they accused Russia of having already sent troops into its territory.
Yatsenyuk, speaking Saturday at a Cabinet meeting, called the Russian presence in Crimea "nothing but a provocation."
But, he said, it failed.
"Ukraine will not be provoked, we will not use force, we demand that the government of the Russian Federation immediately withdraw its troops and return to their home bases," he said.
Ukraine on Friday accused Russian Black Sea forces of trying to seize two airports in Crimea but said Ukrainian security forces had prevented them from taking control.
Groups of armed men, dressed in uniforms without identifying insignia, patrolled the airports in the regional capital, Simferopol, and the nearby port city of Sevastopol.
The men remained at the airports Saturday and Yevgey Plaksin, director of the airport in the regional capital, Simferopol, said Crimean airspace would remain closed until evening.
But by 6:30 p.m. Saturday, the airspace had reopened, and airport services were working, Plaksin said.
Russia's ambassador to the United Nations, Vitaliy Churkin, said Friday that the reports of Russian troops taking charge of positions on the ground were rumors and noted that rumors "are always not true."
"We are acting within the framework of our agreement," he said.
Russia has been conducting a military exercise near its border with Ukraine -- snap drills that Moscow announced Wednesday.
Obama: Warning to Russia
The United States urged Russia on Friday to pull back from the Crimea or face consequences.
In comments from the White House, U.S. President Barack Obama that any violation of Ukraine's sovereignty and territorial integrity would be "deeply destabilizing," and warned "the United States will stand with the international community in affirming that there will be costs for any military intervention in Ukraine."
His message was heard in Moscow, where the head of Russia's upper house of Parliament said Saturday that she planned to ask Putin to recall Russia's ambassador to the United States. Valentina Matvienko, chairwoman of the Federal Council, cited "the recent statements by the U.S. President threatening Russia."
The message also reached Congress, where the ranking member of the Senate Armed Services Committee called for an immediate response to Russia's move.
"Every moment that the United States and our allies fail to respond sends the signal to President Putin that he can be even more ambitious and aggressive in his military intervention in Ukraine," Sen. John McCain, R-Arizona, said in a statement.
He called on Obama to "make clear what costs Russia will face for its aggression and to impose those consequences without further delay."
On Saturday, a U.S. official told CNN's Barbara Starr that Defense Secretary Chuck Hagel had spoken with his Russian counterpart about the crisis.
UK Foreign Secretary William Hague said Saturday that he had asked Russian Foreign Minister Sergey Lavrov for a de-escalation of the situation in Crimea and asked that Russia respect Ukraine's sovereignty and independence.
He called the vote by Russia's parliament "a potentially grave threat to the sovereignty, independence and territorial integrity of Ukraine and said the British Foreign Office had summoned Russia's ambassador to Britain.
He said he planned to visit Ukraine on Sunday to meet with government leaders there and to offer "the UK's support for the territorial integrity of Ukraine."
Other world leaders joined an international outcry, with EU High Representative Catherine Ashton deploring Russia's "unwarranted escalation of tensions."
The U.N. Security Council met Saturday afternoon -- for the second consecutive day -- to discuss the situation.
A statement from the spokesperson for Secretary General Ban Ki-moon said he would speak "shortly" with Putin and called "for an immediate restoration of calm and direct dialogue."
Ukrainian leaders and commentators have compared events in Crimea to what happened in Georgia in 2008. Then, cross-border tensions with Russia exploded into a five-day conflict that saw Russian tanks and troops pour into the breakaway territories of South Ossetia and Abkhazia, as well as Georgian cities. Russia and Georgia each blamed the other for starting the conflict.
Former Georgian President Mikheil Saakashvili told CNN he had warned in 2008 that Ukraine would be next. "Putin is following his blueprint all the way through," he said.
That comparison was noted by Yulia Tymoshenko, who opposed ousted pro-Russian President Viktor Yanukovych. "They want a war like the one which happened in Abkhazia and Ossetia," she said in a statement on her Fatherland party website.
Ukraine, a nation of 45 million people sandwiched between Europe and Russia's southwestern border, has been plunged into chaos since the ouster a week ago of Yanukovych following bloody street protests.
Yanukovych resurfaced Friday in the southwestern Russian city of Rostov-on-Don, where he told reporters that he had not been overthrown and vowed to fight on for Ukraine's future, but gave little indication that he had the support to do so.
Ukraine's new government faces challenges that go beyond Crimea -- the country is an economic basket case.
A $15 billion loan offer from Russia, extended in November after Yanukovych dropped the EU deal, is on hold.
Russia also promised to slash natural gas prices. However, Russian energy giant Gazprom said Saturday that Ukraine is $1.55 billion in arrears on payments for natural gas deliveries, which may force the firm to cancel the discount it agreed to last year, Russian state-run RIA Novosti news agency said.
Gazprom spokesman Sergei Kupriyanov said that Russia has issued a $3 billion line of credit to Ukraine to help it cover its gas debts -- but that payment obligations must be respected.
Ukrainian authorities have said they will need $35 billion in foreign funds by the end of 2015.
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