German Chancellor Angela Merkel said a referendum announced by Crimea's parliament on Thursday on joining the Russian Federation was "illegal and incompatible with Ukraine's constitution."
Speaking after an emergency meeting of European leaders in Brussels, Merkel said: "we condemn the violation of Ukraine's sovereignty with regard to Crimea and we consider its territorial integrity to be essential."
EU leaders urged Russia to immediately withdraw troops from Crimea and said the EU had suspended bilateral talks on visas with Moscow.
Crimea's parliament voted to join Russia on Thursday and its Moscow-backed government set a referendum on the decision in 10 days' time in a dramatic escalation of the crisis over the Ukrainian Black Sea peninsula.
Reporting by Erik Kirschbaum and Alexandra Hudson; Editing by Noah Barkin