MOSCOW (AFP) - US President Barack Obama told President Vladimir Putin that Russia was flouting international law and risking political isolation by its military incursion into Ukraine's Crimea region, as international condemnation increased.
The 90-minute telephone call between the two leaders - the kind of direct confrontation between the White House and the Kremlin rarely seen since the Cold War - came after the Russian parliament gave Putin the green light to send troops into neighbouring Ukraine.
Ukraine's interim leaders immediately responded by both putting its army on heightened alert and voicing confidence that war will be averted because it would break the two neighbours' historic relations for good.
The North Atlantic Treaty Organisation called emergency talks with its 28 ambassadors for Sunday at 1200 GMT (8pm Singapore time) to discuss the escalating crisis.
Ukraine's Foreign Minister Andriy Deshchytsya said Kiev had appealed to Nato "with a request to consider all options to defend the territorial integrity and sovereignty of Ukraine", according to the Interfax news agency.
British Foreign Secretary William Hague said ahead of a visit to Kiev on Sunday that he had summoned the Russian ambassador to register his concerns over Moscow's "potentially grave threat to the sovereignty, independence and territorial integrity of Ukraine".
Mr Obama told Mr Putin that his actions were a "breach of international law, including Russia's obligations under the UN Charter, and of its 1997 military basing agreement with Ukraine".
US officials have warned that Mr Obama and other European leaders were unlikely to show up at the G8 summit in the showcase Black Sea resort of Sochi if the Ukraine crisis was not resolved.
"Going forward, Russia's continued violation of international law will lead to greater political and economic isolation," the White House warned in a statement.
Condemning the "unwarranted escalation of tensions", EU foreign affairs chief Catherine Ashton said the 28-nation bloc's foreign ministers would hold crisis talks on Monday.
US Secretary of State John Kerry told Moscow it was risking regional peace and security.
"The effect on US-Russian relations and on Russia's international standing will be profound," Kerry said in a statement.
Mr Kerry also revealed he had held a joint conference call earlier in the day with his global counterparts "to coordinate on next steps".
A bloody three-month uprising in the former Soviet republic swept pro-EU leaders to power in Kiev, while also sparking a pro-Kremlin backlash on the Crimean peninsula.
In a stark escalation of what threatens to become the worst crisis in relations between Moscow and the West since the Cold War, Kalashnikov-wielding militia hoisted the Russian flag over Crimean government buildings and seized control of the peninsula's airports.
Pro-Kremlin rallies also swept several big eastern and southern Ukrainian cities whose cultural links to Moscow stretch back centuries and whose economic survival depends largely on Russian trade.
Mr Putin's shock decision to seek authorisation from the upper house of parliament to use force in the country of 46 million came less than a day after Obama warned that any such action would carry "costs" for Moscow.
In his request for the use of force, Mr Putin said Russia also had to protect servicemen of the Black Sea Fleet that is based in Crimea's port town of Sevastopol "fully in line with an international accord".
The Federation Council unanimously approved Putin's request after a lightning-fast debate.
The size of the force was not immediately clear.
Mr Putin had issued only one brief statement since Ukraine's parliament on February 22 ousted president Viktor Yanukovych - who has since fled to Russia - after a week of carnage in Kiev that claimed nearly 100 lives.
In his call with Mr Obama, Mr Putin said Russia had the right to "protect its interests" in Ukraine, as he warned of a "real threat weighing on the lives and the health of Russian citizens" in the country, a Kremlin statement said.
Russia's step towards its first war since a five-day conflict with neighbouring Georgia in 2008 sparked an immediate international outcry.
At the United Nations, the United States demanded an immediate end to Russian intervention in Ukraine as Kiev urged the Security Council to stop the crisis.
During emergency talks of the 15-member council, US envoy Samantha Power called for monitors from the UN and the Organization for Security and Co-operation in Europe to be deployed to Ukraine and backed a separate international mediation mission to Crimea.
"It is time for the Russian intervention in Ukraine to end. The Russian military must stand down," she said.
In a call with Mr Putin, a "gravely concerned" UN Secretary General Ban Ki Moon urged him to quickly organise direct talks with authorities in Kiev.
"Cool heads must prevail," the UN chief added.
The vast country's bloodiest crisis since its 1991 independence erupted with Yanukovych's decision in November to reject a deal that would have opened Ukraine's door to eventual EU membership in favour of tighter ties with old master Moscow.
Ukraine's interim president Oleksandr Turchynov told the nation he had put the armed forces on alert and ordered extra security around airports and nuclear power plants.
Ukraine's Defence Minister Igor Tenyukh had earlier said that Russia had already sent 30 armoured personnel carriers and 6,000 additional troops into Crimea to help pro-Kremlin militia gain broader independence from the new pro-EU leaders in Kiev.
Several dozen people were hurt in the eastern city of Kharkiv when a few hundred people broke away from a pro-Russian crowd of 20,000 and attempted to storm the regional administration centre.
More than 10,000 people carrying Russian flags also protested against Kiev's new rulers in the ousted leader's eastern stronghold of Donetsk.
A top Moscow senator said 143,000 Ukrainians had fled to Russia since the crisis first broke out.
In Mr Obama's separate talks with French President Francois Hollande and Canadian Prime Minister Stephen Harper, the leaders pledged to work together on an aid package to the hard-hit country.