Russia's parliament on Saturday gave President Vladimir Putin the go-ahead to send troops into Ukraine, despite a warning from Washington that such a deployment would results in "costs" for Moscow.
The stark escalation of the ex-Soviet country's three-month political crisis came amid growing instability in Ukraine's predominantly Russian peninsula of Crimea that has housed Kremlin navies for nearly 250 years.
Large swathes of the rugged Black Sea peninsula of nearly two million people are now under the control of pro-Kremlin militia who have hoisted the Russian flag over the region's government buildings and seized control of airports as well as television centres.
Putin had remained silent since Ukraine's parliament ousted pro-Kremlin president Viktor Yanukovych on February 22 and then appointed a new pro-Western government that aims to move the ex-Soviet nation of 46 million closer to the European Union.
But the Kremlin said in a statement on Saturday that Putin had asked Russia's upper house of parliament to authorise the use of force in Ukraine until the political situation there "normalised".
"In connection with the extraordinary situation in Ukraine and the threat to the lives of Russian citizens... I submit to the Federation Council a request to use the armed forces of the Russian Federation on Ukrainian territory until the normalisation of the political situation in that country," the Kremlin quoted Putin as saying in the document.
Putin said that Russia also had to protect servicemen from its Black Sea Fleet which is based on the Ukrainian Black Sea peninsula of Crimea "fully in line with an international accord".
The Federation Council unanimously approved Putin's requested a lightning-fast debate.
Upper chamber chair Valentina Matviyenko also ordered the Council's foreign affairs committee to ask Putin to recall the Russian ambassador from the United States.
There was no immediate indication about the number of troops Putin intended to send.
Ukraine's Defence Minister Igor Tenyukh had earlier told the new government's first cabinet session that Russia's armed forces had sent 30 armoured personnel carriers and 6,000 additional troops into Crimea in a bid to help local pro-Kremlin militia gain broader independence from the new pro-EU leaders in Kiev.
Tenyukh accused Russia of starting to send in these reinforcements on Friday "without warning or Ukraine's permission."
The ex-Soviet country's bloodiest crisis since its 1991 independence erupted in November when ousted president Viktor Yanukovych -- who has since fled to Russia -- rejected an historic deal that would have opened Ukraine's door to eventual EU membership in favour of tighter ties with old master Moscow.
The move triggered mass anti-Yanukovych protests and a week of carnage in Kiev claimed nearly 100 lives last week.
Putin's move came after an appeal for help from Crimea's newly-chosen premier Sergiy Aksyonov -- a ruler not recognised by Kiev and appointed by regional lawmakers after gunmen had seized the parliament building in the regional capital Simferopol on Thursday.
"I ask Russian President Vladimir Putin to help in ensuring peace and calm on the territory of Crimea," Aksyonov said in an address broadcast in full by Russian state television.
Aksyonov also said that all of Crimea's security forces -- including the regional armed forces and police -- would now be subordinate to him.
His appeal was immediately picked up by top legislators in Russia's two houses of parliament.
Matviyenko suggested sending in a "limited contingent" -- a phrase that mimicked the language used for the Soviet Union's invasion of Afghanistan in 1979.
In response, Ukraine's boxer turned politician Vitali Klitschko urged parliament to ask interim president Oleksandr Turchynov to declare a "national mobilisation after the start of Russian aggression against Ukraine."
The UDAR (Punch) party leader also asked the UN Security Council to hold an urgent session to discuss the crisis.
US President Barack Obama had warned Putin on Friday that "there will be costs for any military intervention in Ukraine."
"We are now deeply concerned by reports of military movements taken by the Russian Federation inside of Ukraine," Obama said in a hurriedly scheduled statement at the White House.
A senior US official separately told AFP that Obama and some key European leaders could skip June's G8 summit in Sochi if Moscow's forces became more directly involved in Ukraine.
Putin's request for force authorisation came after a flurry of diplomatic efforts to resolve what threatens to become the most dire crisis to hit Moscow's relations with the West since the Cold War.
British Foreign Secretary William Hague said ahead of his arrival in Kiev on Sunday that he had urged a "de-escalation in Crimea and respect for the sovereignty and independence of Ukraine" in a telephone conversation with Russian Foreign Minister Sergei Lavrov.
Germany and France also voiced concern about the developments in Crimea.
And Sweden's Foreign Minister Carl Bildt tweeted: "Obvious that there is Russian military intervention in Ukraine. Likely immediate aim is to set up puppet pro-Russian semi state in Crimea".
Kiev's newly installed leaders have grappled with the dual threats of economic collapse and secession by regions that had backed Yanukovych.
The looming threat of a debt default that Kiev leaders warn could come as early as next week looked even more ominous when Russia's state-owned Gazprom -- often accused of being wielded as a weapon by the Kremlin against uncooperative ex-Soviet states -- warned that it may be forced to hike the price it charges Ukraine for natural gas.
"The debt is $1.549 billion, it is huge," Gazprom spokesman Sergei Kupriyanov told the RIA Novosti news agency.
"Clearly, with this debt Ukraine may not be able to keep its discount (to market price) for the gas."
Ukraine won a one-third discount from Gazprom under a deal signed by Yanukovych with Putin that also saw Russia promise to buy $15 billion in the Kiev government debt.
But Russia has only bought $3.0 billion in Ukrainian obligations and has effectively frozen further deliveries of aid.
Ukraine's new leaders have said that the country needs $35 billion over the coming two years to keep the economy afloat.