The group's 28 leaders, including Prime Minister David Cameron, last night threatened "serious consequences" unless President Vladimir Putin rolls back on the crisis after he sent his Russian troops into the area this week.
They met for lengthy talks at an emergency meeting of the EU in Brussels after Crimean MPs voted to join Russia. The Moscow-backed parliament also announced a referendum on March 16. Two million people will get the chance to decide whether the region should secede from Ukraine and join Russia.
The EU leaders agreed to suspend visa and investment talks with Russia and announced Russian assets would be frozen and said the EU would withdraw from the G8 summit in Sochi in June unless Russia reversed course.
After six hours of talks to discuss how to respond to Russia's manoeuvres against Ukraine, the European leaders said in a joint statement: "We have today decided to take actions ... notably to suspend bilateral talks with the Russian Federation on visa matters as well as talks with the Russian Federation on the new agreement" on investment and research cooperation.
They also backed Britain, France, Germany and Italy, which have broken off from preparations for the G8, with the possibility of calling off participation altogether.
European Council President Herman Van Rompuy said: "The solution to the crisis should be found through negotiations between the governments of Ukraine and the Russian Federation, including through potential multilateral mechanisms.
"Such negotiations need to start within the next few days and produce results within a limited time frame."
Diplomats said the EU's aim was to set out a three-step process, with sanctions pressure steadily increasing if Russia does not respond and start to engage in mediation. The EU will consider imposing travel restrictions on Russian officials, an arms embargo and asset freezes.
Mr Van Rompuy added: "Any further steps by the Russian Federation to destabilise the situation in Ukraine would lead to severe and far-reaching consequences for relations between the European Union and its member states on the one hand, and the Russian Federation on the other, which will include a broad range of economic areas.
Mr Cameron said there was "no excuse now" for the Russians not to meet the Ukrainians for talks.
He added: "If the talks don't get under way and the talks don't make rapid progress, then we move to the second stage, which is asset freezes and travel bans and that again could happen relatively quickly if progress isn't made.
"All the offers last night were on the table about how to create a contact group that includes Ukraine and Russia. That's clearly what needs to happen, it's less important who else is in that contact group.
"What matters is that, with international support, the Ukrainian government and the Russian government are talking to each other. Frankly, there is no excuse now for that not to happen and the Russians need to help make that happen, and if they don't, consequences will follow."
US President Barack Obama, who earlier imposed travel bans on a number of Russian officials - but not Mr Putin - said: "The proposed referendum on the future of Crimea would violate the Ukrainian constitution and violate international law. Any discussion about the future of Ukraine must include the legitimate government of Ukraine."