Ukraine today accused Russia of pouring extra troops and military planes into Crimea as world leaders grappled with the worst standoff between Moscow and the West since the Cold War.
Russian Foreign Minister Sergei Lavrov said the troops were needed in the flashpoint Black Sea peninsula until "the stabilisation of the situation" in Ukraine, and criticised the West for its threats of "sanctions and boycotts".
Crimea -- a strategic Black Sea peninsula with a majority ethnic Russian population -- has been under de facto occupation by pro-Kremlin troops since President Vladimir Putin won parliament's authorisation Saturday to use force in Ukraine.
The price of oil surged on fears of an all-out conflict as the Kremlin looked set to send troops into eastern Ukraine -- a vast industrial region with close ties to Russia that Putin has vowed to protect from "ultranationalist forces".
British Foreign Secretary William Hague warned Russia of "consequences and costs" as he met Ukraine's Western-backed but untested interim leaders in Kiev.
Already, the world's richest nations have threatened to strip Moscow of its coveted seat at the Group of Eight for menacing its ex-Soviet neighbour.
But Europe and Washington appear to have limited options in dealing with Putin -- a veteran strongman with mass domestic appeal who has cracked down on political freedoms and appears more interested in rebuilding vestiges of the Soviet Union than repairing relations with the West.
Ukraine has soared to the top of the global agenda even as the brutal conflict in Syria rages and talks on Iran's nuclear drive enter their most sensitive stage.
"This cannot be a way in the 21st century to conduct international affairs," Hague told reporters. "It is not an acceptable way to behave and there will be consequences and costs."
The crisis on the eastern edge of Europe threatens to blow up into the biggest test to global diplomacy since the fall of the Berlin Wall.
It first erupted in November when protests began against the pro-Kremlin regime over its scrapping of an EU pact and culminated in a week of carnage last month that claimed nearly 100 lives and saw the downfall of president Viktor Yanukovych.
"There was the (1962) Cuban missile crisis and the Soviet Union's decision to send tanks into Prague (in 1968). But in that era, we were effectively in a state of war," said Alexei Malashenko of the Carnegie Moscow Centre.
Germany offered a rare glimmer of hope by announcing that Putin had agreed in telephone talks with Chancellor Angela Merkel Sunday to set up a contact group on Ukraine. Western allies in NATO also said they wanted to send international observers to Ukraine while engaging Moscow in direct talks.
