The European Union widened its sanctions over Russia's behaviour in Ukraine on Monday, adding 13 individuals and two Crimean companies to its sanctions list.

The new penalties will be enforced in addition to those already in place on 48 Russians and Ukrainians with close ties to the Kremlin, who were targeted by the bloc after Russia annexed the Crimea region in March.

Foreign ministers attending the Brussels meeting called Sunday's rebel-organised referendum in east Ukraine "illegal" and lacking in credibility.

"These attempts at referendums have zero credibility in the eyes of the world, they are illegal by anybody's standards," British Foreign Secretary William Hague told journalists.

Before the meeting, Dutch Foreign Minister Frans Timmermans told reporters that "we need to send a clear message that what we have seen until now is completely unacceptable."

Dependent on Russia for energy supplies, the EU has been divided over the severity of sanctions it should impose on Russia for its behaviour in Ukraine.

The bloc has previously imposed asset freezes and travel bans on 48 Russians and Ukrainians after Russia annexed Crimea, but European leaders have so far avoided sanctions against high-profile Russian companies or sectors of the Russian economy.

The EU relies on Russia for around a third of its gas needs, 40% of which arrives via Ukraine. Hawks including Britain, France and Poland have been pushing for harsher penalties on Moscow while Italy, Greece, Cyprus and Spain have remained dovish.