Balkans countries could start joining the EU in 2025 but only if they resolve all outstanding border disputes, European Commission chief Jean-Claude Juncker said on Tuesday.
Juncker said it was merely an "indicative" date for Serbia and Montenegro, the first of six countries in the region covered by a European Commission report on enlargement of the bloc that is due to be released later Tuesday.
"There will be no further accession of the countries of the Western Balkans without resolving the main conflicts beforehand," Juncker told the European Parliament in Strasbourg, France.
"This gamble of saying that we will solve the problem some time after accession, will not take place a second time, not with me."
The 28-nation EU is keen to expand into what it regards as its backyard in the Balkans to prevent other regional actors including Russia and Turkey increasing their influence there.
But it has told those wanting to join that they must first deal with issues involving corruption and the rule of law, as well as territorial disputes.
The Balkans has long been a European flashpoint, most recently with the break-up of the former Yugoslavia.
Serbia and Kosovo remain at odds nearly 20 years after the war that led to Kosovo's independence, while a quarrel between Macedonia and current EU member Greece over Macedonia's name saw huge protests in Athens on Sunday.
The EU has also been stung in the past by taking on new members without having resolved all possible problems, for example a current maritime dispute between Croatia, which joined in 2014, and Slovenia.
Juncker said last year that he wanted Serbia and Montenegro to join by 2025 but warned on Tuesday against too much "excitement" over the prospect.
"It is wrong if it is claimed that I or the Commission have said that Serbia's and Montenegro's accession must be in place by 2025," Juncker told MEPs.
"No, this is an indicative date, an encouragement date, so that the people concerned can consistently set out on their way."
EU leaders will hold a summit in Bulgaria in May with the leaders of the six Balkan nations involved in the report.
Juncker ruled out any enlargement during his five-year tenure when he took the helm of the EU's executive arm in 2014 - and in fact the bloc is set to lose a member, Britain, next year.