European interior ministers convened in Brussels on Thursday to discuss how the bloc will coordinate its latest security and migration policies with Western Balkan states.

The meeting came following the European Commission's strategy for the Western Balkans, published last month, which formally outlined the EU's hope that the remaining nonmember states in the region will be ready to join the bloc by 2025. These include Serbia, Montenegro, Bosnia-Herzegovina, Kosovo, Macedonia and Albania.

"Accession is not a dream but a reality, a reality that is going to come," Juncker told reporters in Sofia. "But more importance needs to be paid to the substance than the calendar."

German Interior Minister Thomas de Maiziere came out against naming a specific day for the Balkan nations to join the EU.

"I do not think much of naming a date and saying by 2025 everything has to be finished," he said. "The right way would be for states that want to join the EU to fulfill the requirements on their own. Then a decision can be made on whether they are ready for the EU."

Read more: EU expansion: Juncker stresses real progress on western Balkans trip

Security is one of the six flagship initiatives in the Commission's strategy.

According to Brussels, notable areas include organized crime and terrorism, including radicalization leading to violent extremism, the challenge of foreign terrorist fighters and the trafficking of firearms and explosives.

All the security recommendations drafted on Thursday are expected to be formalized into policy goals at the upcoming EU-Western Balkans summit in Sofia on May 17.

"I will be returning to Brussels with the conviction, which I already have in fact, that the place of the Western Balkans is at the heart of the European Union," Juncker said.