European Union leaders jointly warned Russia on Thursday, saying Moscow would face "massive consequences" if it "invades" Ukraine, which could see the 27 nation bloc coordinating with allies to take measures against Russia.

The leaders adopted a declaration that said Moscow "must de-escalate tensions caused by the military build-up," which they claim it ordered along the border with Ukraine. The EU urged Moscow to engage in diplomatic talks through an existing forum with Paris, Berlin, and Kiev.

An alleged invasion would bring a "severe cost in response, including restrictive measures coordinated with partners," the summit conclusions said as reports suggest that the United States was urging the Europeans to prepare a joint riposte against Moscow.

The EU's warnings came amid western claims of a troop buildup on the border with Ukraine and preparing for a military invasion - accusations Russia has long been denying.

The West had previously threatened Russia with "heavy sanctions," but President Vladimir Putin said earlier this month Moscow pursues a peaceful foreign policy; however, it has the right to defend its security.

The European Union is trying to uphold influence on several eastern European countries that once were Soviet republics through a Wednesday summit.

The Eastern Partnership summit began Wednesday, and before that, EU and national leaders had side meetings on some regional issues.

Russian President Vladimir Putin said Tuesday he wanted "immediate" talks with the United States and NATO over security guarantees amid the rising tensions between Russia, Ukraine, and the West.