Ukraine would need at least six years to prepare itself for accession into the European Union and NATO, the country's President Petro Poroshenko declared on Monday.

He added membership would be put forward on a national referendum so that citizens could have the final say.

During a joint press conference with his Lithuanian counterpart Dalia Grybauskaite, who is on a visit to Kiev, Poroshenko said Ukraine would need that time to cover the criteria for both organizations.

The President made clear a decision whether or not Ukraine should join the alliance was "within the exclusive competence of the Ukrainian people."

Ukraine has signed agreements obliging it to conform to NATO standards by 2019. However, no existing accord provides for its accession in the alliance.

On Sunday German Foreign Minister Frank-Walter Steinmeier spoke firmly against the EU and NATO integration of Ukraine.

Steinmeier told Der Spiegel magazine that a EU membership of Ukraine in the short term is "unrealistic". In his words, political and economic modernization in Ukraine is a matter of "a few generations".

At the same time NATO Secretary General Jens Stoltenberg sad at the 60th Plenary Session of the NATO Parliamentary Assembly that the alliance's door "remains open" for Ukraine, adding NATO respected Kiev's decision to follow a non-alignment policy.

Moscow has repeatedly warned against any attempts at including Ukraine in NATO, with Russian President Vladimir Putin's Spokesman, Dmitry Peskov, requiring from the West to guarantee Ukraine will not become part of the Alliance.

In September Prime Minister Arseniy Yatsenyuk said his country would pursue a pro-NATO path in the long-term.