The declaration was adopted by the leaders of the European Union and of its member states, in consultation with their Western Balkans partners, and in the presence of relevant regional stakeholders, the Bulgarian Presidency of the Council of the European Union said on its website.
"I have no doubts that we will also witness in the future many problems", he said, adding that the European Union was offering no "unrealistic" fast-track to take the six in.
The EU is also wary of admitting new members before they settle their differences, particularly in a region still bedevilled by the aftermath of the bloody break-up of Yugoslavia in the 1990s.
"In my speech, I, of course, expressed the support and attitude of Slovakia to the further enlargement of the European Union, I encouraged the countries of the Western Balkans that have launched and are doing many complicated and hard reforms to bring them closer to the European Union membership" he said, adding that on the other hand he asked member states and European Union leaders to behave correctly to these countries, and that if they are given conditions to join the organisation, they must "see the light at the end of the tunnel".
"No progress was made on enlargement; this is a question that remains for the June EU Summit", German Chancellor Angela Merkel said more bluntly, a few hours earlier.
Greek Prime Minister Alexis Tsipras told reporters that the differences had not been resolved.
"We talked about other infrastructure projects, one of which is the construction of the Nis-Pristina highway".
Polish Prime Minister Mateusz Morawiecki is welcomed by Bulgarian Prime Minister Boyko Borissov in Sofia. The 2015 nuclear agreement is an important element of regional peace and security so we chose to support it regardless of the United States' decision.
"It is the way to use more effectively the time between now and tomorrow".
The six Western Balkan countries are not mentioned by name and are referred to as "partners", as supposed to "states" or "countries", in an apparent effort to ease concerns about the reference to Kosovo's status.
Last, but not least, the annex points to the completion of the Regional Electricity Market that will later be integrated within the EU's own Internal Energy Market. Serbia also signed a memorandum of understanding with the European Investment Bank, as well as issued a joint statement on the construction of the Bulgaria-Serbia gas interconnector.
European Commission President Jean-Claude Juncker has previously said it is the EU's responsibility to maintain peace and stability in the region by offering countries a route into the bloc.