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European Union leaders are encouraging Balkans countries to continue on the path of EU-oriented reform but they are reluctant to say when any of the six might be able to join the bloc.

At a summit in Bulgaria Thursday, the EU's Enlargement Commissioner Johannes Hahn said that "the whole process is merit based. So these countries have to do their homework."

Accession to the EU has been a powerful driver of democratic reform in Albania, Bosnia and Herzegovina, Macedonia, Montenegro, Serbia and its former territory of Kosovo.

Divisions have surfaced over the challenge of migration and with Britain set to leave next year, a group of countries led by France say the EU must itself be reformed before new countries can come aboard.