The European Union’s ambitions for Serbia and Kosovo could be severely compromised if the two historic rivals refuse to normalize relations.

As part of the EU’s plan to integrate the troubled region into the EU, European Union Commission President Ursula von der Leyen issued a star warning on a visit to the Western Balkans.

According to Von der Leyen, Serbia must recognize Kosovo as an independent nation and Kosovo must provide autonomy for ethnic Serbs in northern Kosovo. In practice, this would fulfill Serbia and Kosovo’s previous commitments negotiated earlier this year, but neither side seems committed to implementing them.

As Helena Ivanov, associate fellow at the Henry Jackson Society, noted, statements from such a high political representative carry weight, but it is yet to be seen if they will make a difference.

According to Kosovo Prime Minister Albin Kurti, the Association of Serb Municipalities isn’t really on the table, and Serbia has made it clear that membership in the U.N. is a red line for the country. Ivanov said it is unclear how the EU intends to press the two sides to cross their respective lines.

The majority of Kosovo’s ethnic Albanians are Serbs, who remain loyal to Belgrade and refuse to recognize Kosovo’s 2008 unilateral declaration of independence. It was designed to give the Serb community in Kosovo a significant degree of self-governance through the creation of an Association of Serb Municipalities, a commitment Kosovo made in 2013 as part of the Brussels Agreement.