Tensions in Ukraine's restless east could become full-scale war, with Russian president Vladimir Putin authorising a military advance on the Crimea region.
But the actual text of the bill does not limit troop action to that region, but gives Russian armed forces carte-blanche to enter the whole country, not just the Crimea.
In that peninsula, the majority ethnic Russians have loudly demonstrated their opposition to the Kiev protests that toppled Kremlin-friendly president Viktor Yanukovych, and prefer closer relations with Moscow.
The request was unanimously approved by Russia's politicians in the lower and upper houses on Saturday, within hours of a request from the Kremlin. More than 6,000 armed men, believed by many to be Russian troops but bearing no official insignia, are already in the territory.
In response, an emergency meeting of the United Nations Security Council was convened at the insistence of the UK government, which has also summoned the Russian Ambassador Alexander Vladimirovich to the Foreign Office "to register our deep concerns," Foreign Secretary William Hague said in a statement on Saturday.
And in an unexpected escalation,according to Russian news agency Interfax, Russia's Federal Council deputy Speaker Yuri Vorobyov called for the withdrawal of Russia's ambassador to the US, a motion approved by the House.
"Yesterday from various media, we heard that President Obama said that Russia will pay dearly for their policies. We know that the Maidan militia, who acted in Kiev, were trained in Lithuania and Poland. Now they want to force their will on the eastern regions of Ukraine and Crimea, he said.
"I believe that these words of US President - are a direct threat, and he crossed the red line, he insulted the Russian people," he continued, saying that, in such circumstances it was necessary to withdraw the Russian ambassador from the United States.
Ukraine's acting president Oleksandr Turchynov has called an emergency meeting of security chiefs, in wake of the threat.
The Kremlin said in a statement, prior to the vote: "Vladimir Putin made an appeal to the Council of the Federal Assembly of the Russian Federation.
"Due to the extraordinary situation on Ukraine, with threats to the lives of citizens of the Russian Federation, our compatriots, and of the the personnel of the military contingent of the Armed Forces of the Russian Federation located in accordance with the international agreement on the territory of Ukraine (in the Autonomous Republic of Crimea), and on the basis of Paragraph D, Part 1 of Article 102 of the Constitution of the Russian Federation, I am submitting to the Federation Council of the Federal Assembly of the Russian Federation an appeal for use of the armed forces of the Russian Federation on the territory of Ukraine, for the purpose of normalising the political situation in this country. "
Reports suggest 6,000 Russian troops have been sent into the Ukraine, despite a warnings from Barack Obama that "there will be costs" if the country militarily intervenes in the crisis.
Reporters on the ground described celebrations at the news from the Crimea, the majority of whom are ethnic Russians. Protesters raised Russian flags and beat up supporters of the new Ukrainian government, according to AP.
The pro-Moscow leader of Crimea, Sergiy Aksyonov, had earlier appealed to Putin for help to keep the peace, which Putin said "had not gone unnoticed".
US officials said have said Obama may cancel plans to attend an international summit in Russia this year and could halt discussions on deepening trade ties with Moscow.
On Saturday night, the US President warned that "there will be costs" if Russia intervenes militarily, without explaining what those costs might be.
Speaking after the Russian voted to send in troops, Hague said he was "deeply concerned" by the decision.
He called the move a "potentially grave threat to the sovereignty, independence and territorial integrity of Ukraine" and said the UK condemned any act of aggression against Ukraine.
Hague, who will visit Kiev tomorrow, said he had reiterated that Britain supports the Ukrainian Government and had now summoned the Russian Ambassador to the Foreign Office.
Both the UK and Germany have issued a joint call for "international diplomatic action to address the crisis," Hague said. “The EU must agree urgently an asset freezing regime to target those suspected of laundering the proceeds of corruption. On my instructions, the British Embassy in Kiev has told the Ukrainian Government that we stand ready to provide Ukraine with technical advice on asset recovery.”
Hague said he had spoken to Ukraine's acting President and "urged him to ensure that the government takes measures which unify the country, and that it protects the rights of all Ukraine’s citizens, including those from minority groups, in a spirit of inclusiveness".
On Sunday, he will "also discuss how the UK can support the Ukrainian Government in recovering improperly acquired assets."
David Cameron also renewed his call on Moscow to respect Ukraine's territorial integrity in a telephone call with Vladimir Putin yesterday.