Montenegro is the frontrunner to join the European Union. But officials, diplomats and experts fear Belgrade will do everything it can to prevent that from happening, leaving Brussels in a difficult position between the two candidate countries.

While Montenegro closes in on the finish line, Serbia's accession bid looks dead in the water: Belgrade has yet to open all negotiation clusters with the EU, and eight member states blocked its latest attempt last month.

"Serbia's aspirations towards Montenegro are the result of several factors: [Serbian President] Vučić's personal intolerance, the continuity of the promotion of the so-called Serbian world policy against the policy of European integration, and dissatisfaction with the failure of all attempts to bring Podgorica under Belgrade's control," MEP Tonino Picula (Croatia/S&D) told Euronews.

"Basically, it is a programmed state policy that stems from Vučić's multiple frustrations related to Montenegro, from the restoration of Montenegrin statehood and separation from Serbia twenty years ago to the unpleasant fact that Montenegro is apparently entering the final phase of joining the European Union," Picula added.

Three officials working for the European Commission and national governments acknowledged they were aware of these attempts to interfere with Montenegro's internal affairs — a unique complication for the EU.