SIMFEROPOL, Ukraine (CNN) — Russia's Parliament signed off Saturday on President Vladimir Putin's request to send military forces into Ukraine, raising the stakes in a quickly escalating game of brinksmanship between the two countries.
Putin cited the "extraordinary situation in Ukraine" in his request, adding that the lives of Russian citizens and military personnel based in southern Crimea had been threatened, a claim Ukrainian officials have vehemently denied.
The move prompted world diplomats to call for a de-escalation of tensions that have put the two countries on a possible path to war and roiled relations between Russia and the United States.
Ukraine acting President Oleksandr Turchynov took to the airwaves late Saturday to warn any Russian military intervention would lead to war, even as acting Prime Minister Arseniy Yatsenyuk said that he had been told by Russian Prime Minister Dmitry Medvedev that no decision had been made on whether Moscow would dispatch forces.
Yatsenyuk said his country was ready to mobilize its forces to protect strategic locations, including nuclear power plants.
The crisis raised alarm bells with the worlds diplomats, and an emergency meeting of the U.N. Security Council was underway Saturday afternoon where Ukraine's ambassador Yuriy Sergeyev called on the member nations to take a stand against what he called Russia's "clear act of aggression.''
"...The troops are already there, and their number is increasing every hour," he said.
Russia now has 15,000 troops in Ukraine's Crimea region, Yegor Pyvovarov, the spokesman for the Ukraine mission at the United Nations, told CNN ahead of Saturday's session of the Security Council. He did not say how Ukraine arrived at that number, or whether that included troops already stationed at a Russian base in the region.
Vitaly Churkin, Russian ambassador to the U.N., rejected Ukraine's calls to stop Russian intervention. "We can't agree with this at all," he said
He blamed members of the European Union for causing the mass street demonstrations in Ukraine that led to the ouster of President Viktor Yanukovych last week.
"It's a difficult situation in the past few hours," he said, adding that there were Ukrainian forces from Kiev en route to to overthrow the local pro-Russian governments in eastern Ukraine and Crimea and establish new ones that would enforce power from the new provisional government.
The United States and other nations have called on Russia to pull back from the Crimea or face consequences.
In comments Friday 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 Federation Council, cited "the recent statements by the U.S. President threatening 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.
Meanwhile, Ukraine Defense Minister Igor Tenyukh said his nation's military was at its highlight state of military readiness. 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 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.
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
Meanwhile, Obama's message to Russia 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.
-- CNN's Victoria Eastwood and Diana Magnay reported from Simferopol, Ukraine, Ingrid Formanek and Victoria Butenko from Kiev, while Chelsea J. Carter in Atlanta and Laura Smith-Spark in London wrote. CNN's Tom Watkins, Alla Eshchenko, Arkady Irshenko, Radina Gigova and journalist Azad Safarov contributed to this report.