French Foreign Minister Bernard Kouchner said Russia, after having recognized the Georgian breakaway provinces South Ossetia and Abkhazia as independent states, may now set its eyes on "different targets."
Since Russia had violated international law, it might now target "the Crimea, Ukraine, Moldova," he said Wednesday.
German Foreign Minister Frank-Walter Steinmeier warned of a spreading crisis.
"We have to do all in our power so that the crisis in the southern Caucasus does not evolve into a crisis for the entire Black Sea region," he told German daily Sueddeutsche Zeitung in an interview.
Ukrainian officials are concerned that Russia may reclaim the Crimea, handed over to Ukraine by the Soviet Union in 1954. It is home to some 2 million people, the majority of whom are Russians. The Russian navy's Black Sea Fleet anchors in Sevastopol.
British Foreign Secretary David Miliband earlier this week rushed to Ukraine to assemble a "coalition against Russian aggression," assuring support in case Ukraine was being threatened.
Observers say, however, that a direct confrontation over the Crimea is highly unlikely.
EU leaders will discuss the crisis in the Caucasus at a special summit to be held Monday in Brussels.