Western leaders have imposed sanctions and threatened serious consequences against Russia if it fails to deescalate the situation in Ukraine's occupied peninsula of Crimea, where a disputed parliamentary vote to join Russia has heightened fears of conflict.
Ukraine's acting and interim national leaders had already condemned the vote by pro-Russian lawmakers in Crimea as "illegitimate" and have suggested they will block a related referendum.
European Council President Herman Van Rompuy said the EU was suspending visa and cooperation negotiations with Russia and will consider asset freezes and travel bans if the Russian government does not start negotiations with Ukrainian authorities in the "next few days."
He also said the EU will sign an association pact with Ukraine before Ukraine holds elections in May.
But Europe's presidents and prime ministers were divided on more serious steps such as freezing assets and issuing travel bans on Russian officials.
Speaking after an emergency summit of EU leaders to discuss Russia's occupation of Crimea, Van Rompuy said such talks between Moscow and Kyiv to resolve the Ukraine crisis needed to produce results within a "limited time frame."
He said EU leaders had agreed to a three-tiered series of sanctions starting with an immediate suspension of talks.
"The solution to the crisis should be found through negotiations between the government of Ukraine and the Russian Federation, including through potential multilateral mechanisms," Van Rompuy said.
"Such negotiations need to start within the next few days and produce results within limited time frame. In the absence of results, the European Union will decide on additional measures such as travel bans, asset freezes and the cancellation of the EU-Russia summit. The [European] Commission and the European External Action Service will take forward preparatory work on these measures."
A statement issued by EU leaders
called on Russia to immediately withdraw its armed forces to the areas of their permanent stationing and allow immediate access for international monitors to areas in eastern Ukraine.
Van Rompuy also warned Russia of further consequences should Moscow continue to destabilize the situation.
"Any further steps by the Russian Federation to destabilize the situation in Ukraine will lead to severe and far-reaching consequences for relations between the European Union and its member states on one hand and the Russian Federation on the other hand, which will include a broad range of economic areas."
The United States earlier announced the imposition of visa restrictions on individuals and institutions from Russia and Ukraine that it says are "threatening the sovereignty and territorial integrity of Ukraine."
The visa restrictions were announced as U.S. Secretary of State John Kerry headed into a roughly 40-minute meeting with Russian Foreign Minister Sergei Lavrov in Rome, on the sidelines of a gathering to discuss Libya.
Lavrov said afterward Kerry had assured him there are no blacklists of Russian officials as yet. However, Lavrov said any intentions to impose sanctions were already a threat.
The White House also announced that President Barack Obama signed an executive order that authorizes sanctions against "individuals and entities responsible for activities undermining democratic processes or institutions in Ukraine."
Later, Obama said the United States would support moves by European allies to pressure Russia into removing its forces from areas in Crimea. He said that "there is a way out of this that respects the interest of the Russian Federation as well as the Ukrainian people."
He added that the planned referendum called by Crimea's pro-Russian parliament on whether to join Russia was "completely illegitimate." "Any discussion about the future of Ukraine must include the legitimate government of Ukraine," Obama said.
Obama also spoke by phone with Russian President Vladimir Putin. Accoring to the White House, Obama told Putin that Russia's actions in Crimea had violated Ukraine's sovereignty.
Obama outlined the terms of a diplomatic "off-ramp" that U.S. officials are promoting. Under terms of the deal, Russia would pull back troops to bases in Crimea, allow international monitors in to ensure the rights of ethnic Russians are respected, and consent to direct talks with Ukraine officials.
According to the Kremlin, Putin told Obama that Kyiv's new authorities had imposed "absolutely illegitimate decisions on the eastern, southeastern, and Crimea regions." It said that Putin told Obama that Moscow "cannot ignore calls for help in this matter and it acts accordingly, in full compliance with international law."
The Kremlin statement said the conversation revealed "differences in approaches and assessments of the causes which brought about the current crisis." But it said that Putin told Obama that relations should not suffer over Ukraine.
It was the second phone conversation between Obama and Putin in the past six days.
The U.S. House of Representatives, meanwhile, has overwhelmingly passed a bill backing loan guarantees for the new government in Kyiv. The U.S. Senate is expected to consider a similar bill backing $1 billion in loan guarantees next week.
Ukrainian politicians Yulia Tymoshenko and Vitali Klitschko have meanwhile urged Europe to take stronger action over Russia's intervention in Crimea.
Tymoshenko warned that Putin would take advantage of any weakness by the West. Speaking in Dublin, Tymoshenko said Ukraine was too weak to stand up to Moscow alone and that the use of military force against Russia should not be ruled out.
Tymoshenko called for Britain and United States, as signatories of the 1994 Budapest Memorandum that guaranteed Ukraine's territorial integrity, to act as intermediaries for negotiations with Moscow.
Klitschko said Ukrainians were Europeans and pleaded for more European support.
Both were addressing a meeting of the European People's Party, the largest bloc in the European parliament.
Earlier in the day, the parliament in Crimea voted to join Russia and hold a referendum on the topic on March 16 -- far earlier than had previously been suggested.
Announcing the outcome of the March 6 parliamentary vote, Crimea's First Deputy Prime Minister Rustam Temirgaliyev also said Ukrainian armed forces in Crimea will be considered "occupiers" and must surrender or leave.
The Crimean lawmakers' move prompted an angry response from Ukrainian Prime Minister Arseniy Yatsenyuk, who called the vote "illegitimate" and said Ukraine was "prepared to protect our country."
Yatsenyuk called Russia's actions "military aggression" and an "extensive, unacceptable use of Russian military force."
"It is clear that no military option is on the table, but it's clear that it is up to the Russian government to make a step back and to make the first step to stabilize the situation in the region and not escalate further tension between Ukraine and Russia and further tension in the entire Europe," Yatsenyuk, who took office amid crisis on February 27, said.
Yatsenyuk, on a visit to NATO headquarters in Brussels, was told that the North Atlantic alliance "stands by Ukraine's sovereignty [and] integrity."
NATO Secretary-General Anders Fogh Rasmussen, at a joint press conference after talks with Yatsenyuk, said the crisis in Ukraine was "the gravest threat to European security since the end of the Cold War." Rasmussen called on Russia to halt the military escalation in Crimea and to "refrain from any interference elsewhere in Ukraine."
In response to a question, Yatsenyuk said Ukraine was not discussing membership of the alliance, saying, "It is not on our radar."
Ukraine's acting president said the planned Crimean referendum was illegal, describing it as a farce and a crime organized by the Russian military. In a short televised address, Oleksandr Turchynov said late on March 6 that the Ukrainian parliament would start procedures to dismiss the Crimean assembly and block the referendum.
The pro-Russian leadership in Crimea is not recognized by Ukraine’s new central authorities, who have issued arrest warrants for the territory’s prime minister, Sergei Aksyonov, and its parliamentary speaker.
Refat Chubarov, the leader of the Crimean Tatars, said his community will not recognize the results of such a referendum.
Russia's forces have been occupying Crimea since February 28, and Moscow has declared illegitimate the government that emerged after embattled Ukrainian President Viktor Yanukovych fled to Russia.
The Ukrainian government has asked Interpol to issue a "red notice" for the arrest of Yanukovych, who earlier this week made a public appearance in Russia. On its website, Interpol said on March 6 that it was reviewing Ukraine's request.
Some protesters were meanwhile detained when several hundred pro-Russian demonstrators gathered outside the building of the Ukrainian Security Service (SBU) in the eastern city of Donetsk to protest the detention of local pro-Russian leader Pavel Gubarev.
Gubarev reportedly has already been taken to Kyiv
, something that has further angered the crowd, who were reportedly chanting, "Fascists!" at police surrounding the SBU building.
The 100-seat parliament of Crimea on March 6 voted 78-0, with eight abstentions, on joining Russia and the related referendum.
Pro-Russian Crimean politicians have suggested that around 11,000 "self-defense" troops were deployed to help control all access points to the Crimean Peninsula.
Earlier this week, Crimea Prime Minister Aksyonov said "self-defense units" there were armed with "shields and sticks" and many had "legally registered" weapons.
Meanwhile, at least one Ukrainian TV channel has been taken off the air in Crimea. A presenter at 1+1, Lidia Taran, said that reporters at the TV channel had been attacked while working in Crimea.
"Our journalists from the very start of the standoff in Crimea were in danger. Every day our local correspondents are attacked," Taran said. "There were several attempts for our Kyiv crews to drive out and work in Crimea, they were a failure, today one of our crews was taken in."
Other reports said another Ukrainian TV channel, Channel 5, had also been blocked from broadcasting on the Black Sea peninsula.
With reporting by Reuters, AFP, UNIAN, and AP