Kosovo and Serbia on Thursday failed yet again to make progress in talks aimed at improving their long-strained relations, and the European Union’s top diplomat warned that the lack of progress could hurt their hopes of joining the bloc.

EU foreign policy chief Josep Borrell, who supervised the talks in Brussels, blamed the latest breakdown on Kosovo Prime Minister Albin Kurti’s insistence that Serbia should essentially recognize his country before progress can be made on enforcing an agreement they reached in February.

Serbia and its former province of Kosovo have been at odds for decades. Their 1998-99 war left more than 10,000 people dead, mostly Kosovo Albanians. Kosovo unilaterally declared independence in 2008 but Belgrade has refused to recognize the move.