German Chancellor Angela Merkel is leading European Union calls on Ukraine to reform its judicial system beyond the case of jailed former prime minister Yulia Tymoshenko before any association agreement with Kiev is signed.

Merkel on Monday called on Ukraine to take "credible steps" to overcome politically motivated "selective justice" ahead of next week's summit in Vilnius, Lithuania, where the EU is slated to sign a landmark deal with Ukraine.

On top of judicial reform, the 28 EU members also want to see changes in Ukraine's electoral legislation.

Germany has insisted that the continued imprisonment of Tymoshenko is putting the whole project in danger. "It is not yet clear whether Ukraine is willing to create the conditions for a signature," Merkel told the German parliament in Berlin.