European Council President Herman Van Rompuy scheduled a summit of the EU's 28 leaders for Thursday as EU foreign ministers met Monday to assess Russian reaction to Ukraine's new pro-EU leadership in Kyiv.
Rompuy said the summit would "discuss the latest developments in Ukraine and how to facilitate the necessary de-escalation of the situation."
"In the absence of de-escalating steps by Russia the EU shall decide about consequences for bilateral relations of EU and Russia," said the EU ministers in their statement.
German Foreign Minister Frank-Walter Steinmeier stressed the need for mediation, saying talks were needed between Ukraine's new leadership in Kyiv and Russia.
He also called for an observer mission of the Organization for Security and Cooperation in Europe (OSCE) to establish a clear picture of the situation in Crimea and eastern Ukraine.
Visiting Geneva late Monday, Steinmeier had talks with Russian Foreign Minister Sergei Lavrov who criticized threats of "sanctions and boycotts."
Earlier on Monday in Geneva, Lavrov had claimed that Russia was "defending" compatriots in Crimea.
In New York late Monday, the UN Security Council began another emergency meeting on the crisis at the request of Russia which said it wanted to explain its stance.
'Wrong side of history'
From Washington, Obama urged Russian President Vladimir Putin to allow international monitors to mediate in Ukraine, saying Russia was "on the wrong side of history."
"I think the world is largely united in recognizing the steps Russia has taken are a violation of Ukranian sovereignty...a violation of international law," Obama said.
Obama added that the US was also considering economic and diplomatic sanctions.
We are examining a whole series of steps - economic, diplomatic - that will isolate Russia," he said.
State Department spokeswoman Jen Psaki said the US was working closely with its European partners and was looking "to take steps" ranging from sanctions on Russia to economic assistance for Ukraine.
EU: 'clear violation'
The EU foreign ministers in their Brussels statement condemned Russia for what they called "the clear violation of Ukraine's sovereignty and territorial integrity."
"We need to see a return to barracks by those troops that have currently moved [from] where they have been staying," said EU foreign policy chief Catherine Ashton in Brussels (pictured).
"There are serious concerns about over-flights, about reports of troops and armed personnel moving," she said.
Putin watches maneuvers
In his first public appearance for nearly a week, Putin flew on Monday to watch large Russian military maneuvers near St. Petersburg in western Russia. He otherwise remained publicly mute.
Moscow argues that it acted to protect ethnic Russians in Ukraine after last month's uprising by pro-European Ukrainians against their president-turned-fugitive Viktor Yanukovych.
Change course 'extremely fast'
France's Laurent Fabius said he and other EU foreign ministers were ready to suspend visa and cooperation negotiations with Russia if it did not change course "extremely fast" by Thursday's summit.
Germany's Steinmeier was also due to have talks in Geneva with UN chief Ban Ki-moon and Swiss President Didier Burkhalter.
Switzerland currently chairs the OSCE and has already proposed that an OSCE observer mission be sent to Ukraine.
'No military options,' says Ukraine's premier
In Kyiv, Ukraine's new pro-Western interim prime minister, Arseniy Yatsenyuk, appealed for outside help and said "any attempt by Russia to grab Crimea will have no success at all."
During a visit by British Foreign Secretary William Hague, Yatsenyuk said Kyiv had "no military options on the table" to reverse Russia's Crimea moves.
Russian markets tumble
The Moscow stock market fell by 11.3 percent on Monday. Russian gas monopoly Gazprom, which supplies Europe through Ukraine was down nearly 14 percent.
Russia's central bank spent $10 billion (7.2 billion euros) of its reserves to prop up the ruble as investors took fright. The east-west tension also rattled European stock markets, with the DAX closing 3.4 percent in the red in Frankfurt on Monday.
A team from the International Monetary Fund (IMF) arrived in Kyiv late Monday, saying it would conduct a 10-day fact-finding mission to consider the new government's request for urgent financial assistance.
In other retaliatory measures, the US and Britain said they were withdrawing official delegations from the Winter Paralympics that are due to open on Friday in the Black Sea resort of Sochi, where Russia staged the Winter Olympics.
Crimea has 2 million mostly Russian-speaking people and hosts Russia's Black Sea Fleet at Sevastopol.
ipj/kms (AFP, Reuter, dpa, AP)