BELGRADE, Serbia (AP) — Serbia must reach an agreement on normalization of relations with Kosovo if it wants to join the bloc, the European Union's enlargement commissioner said Wednesday.

Johannes Hahn said after meeting Serbia's president, Aleksandar Vucic, that the EU won't import any unresolved disputes with future new member states.

"Serbia needs to conclude and irreversibly implement a legally binding agreement with Kosovo before it can join," Hahn said. "This also should be seen as an opportunity because unresolved border conflicts are never fine for any kind of country."

Hahn visited Belgrade a day after the European Commission released a new strategy paper for the western Balkans region that envisages that Serbia and Montenegro could be the first to join the bloc in 2025 if they meet all necessary conditions and carry out reforms.

The commission said the six Western Balkan EU hopefuls must resolve all disputes in order to be invited in.

Serbia has refused to recognize Kosovo's 2008 declaration of independence. But Belgrade and Pristina have been involved in EU-mediated talks designed to bring the former war foes closer together.

Many Serbs view Kosovo as Serbia's historic heartland and refuse to give up claims to it. Vucic said any future agreement should be the result of a "compromise."

"Serbia won't agree to get nothing while others get everything," he said. "It has to be a compromise where both will lose something before we can start anew."

About 10,000 people were killed during the 1998-99 war in Kosovo which ended when NATO bombed Serbia to end its crackdown against independence-minded ethnic Albanians.

Russia has backed Serbia in its refusal to recognize Kosovo's declaration of independence.