The EU says expanding energy and economic cooperation with Ukraine will in part depend on Kiev's treatment of jailed former Prime Minister Yulia Tymoshenko.

European Council President Herman Van Rompuy said Monday following an EU-Ukraine meeting in Kiev the two sides have agreed on the terms of an "association agreement," which seeks to implement a free trade area and broaden energy and transportation ties.

But, he said, the EU won't yet sign the deal because member states and the European Parliament have deep reservations about "political circumstances" in Ukraine, including "the risks of politically motivated justice."

The imprisonment of Tymoshenko "is the most striking example," Van Rompuy said.

Tymoshenko is serving a seven-year prison sentence after a conviction on charges she abused power when she helped broker a 2009 natural gas deal with Russian energy company Gazprom.

Current Ukrainian officials, including Ukrainian President Viktor Yanukovych, said the deal created long-term economic problems for the country, though she and her Western allies have said the charges are politically motivated.

"The support of European public opinion to Ukraine's political association and economic integration to the EU is an important asset, closely related to this field," Van Rompuy said. "The perceived deterioration of the quality of democracy and rule of law in Ukraine has a direct impact in our member states, in our public at large, and in the European Parliament."

The EU-Ukraine Association Agreement and its Deep and Comprehensive Free Trade Area are seen as vital steps in Ukraine's economic integration into Europe.

It seeks to strengthen democratic institutions, human rights and the rule of law in the country, as well as improve its public finances and reform its electricity tariffs and gas pricing measures, thus encouraging investment in its energy sector.

The EU president said a "litmus test" for Ukraine's commitment to political openness will come with next year's parliamentary elections, which he warned must measure up to the fairness standards established by the Organization for Security and Cooperation in Europe.

"The conduct of these elections will have to meet the commitments of the OSCE, including the ensuring of a level playing field for all possible candidates," he said. "And it is of utmost importance that they can exercise their political rights."

The failure of the sides to sign the association agreement at Monday's summit brought a call for Yanukovych's resignation from a political opposition leader.

Andrii Parubii, a member of Parliament from the Our Ukraine-People's Self Defense Bloc, told local television the lack of an agreement showed Yanukovych isn't committed to European-style democracy and is unfit to lead the country, the Ukrainian News Agency reported.

"A failure to share these values has led to the fact that Ukraine and the European Union have not signed the agreement on political association and a Deep Comprehensive Free Trade Agreement today," Parubii said.

"The responsibility for this falls on Yanukovych as the person with unlimited concentration of power. And I think the only right decision for him is to come up with a New Year's greeting, apologize to the Ukrainian people, say he failed his presidency and to resign," he added.