BRUSSELS (AP) — The European Union’s top court announced Friday that it cannot rule on a decades old border dispute between member countries Slovenia and Croatia because the row falls under international law and not EU law.

Croatia and Slovenia proclaimed independence simultaneously from Yugoslavia in June 1991. Almost three decades later, and after years of negotiations, they still cannot agree on where the border between them lies.

Now, Slovenia’s appeal to the European Court of Justice has failed too.

The Luxembourg-based court said that it “lacks jurisdiction to rule on a border dispute between Slovenia and Croatia” and urged them “to strive sincerely to bring about a definitive legal solution to the dispute consistent with international law.”

Slovenia had argued that Croatia violated EU laws when it refused to implement a 2017 international arbitration ruling in the long-running border standoff. Slovenian officials insist on the implementation of that border arbitration decision and have said they will not re-negotiate the issue with Croatia.