The referendum in Crimea is "illegal and illegitimate" and its outcome "will not be recognised," said a joint statement issued Sunday by European Council President Herman Van Rompuy and European Commission President Jose Manuel Barroso.

The European Union considers the holding of the referendum on the future status of Ukraine's Crimea as "contrary to the Ukrainian Constitution and international law," the statement said.

"Only working together through diplomatic processes, including direct discussions between the Governments of Ukraine and Russia, can we find a solution to the crisis," the statement said.

The EU ministers of foreign affairs will evaluate the situation Monday in Brussels and decide on "additional measures" in line with the declaration of the Heads of State and Government of the European Union of March 6.

A referendum was held in Crimea, an autonomous republic of Ukraine, on the region's future status on Sunday.