Leaders of Serbia and Kosovo have been in last-ditch talks to resolve a dispute over vehicle number plates.
Tensions between the two ratcheted up last year after Kosovo demanded that Serbian cars crossing the border buy temporary licence plates.
This was in response to Serbia already requiring vehicles from Kosovo to do the same.
A temporary deal was struck to allow vehicles to cross the border if state symbols on the number plates were covered up.
But that expired this week.
Ahead of talks between Serbia and Kosovo, the EU's mediator Miroslav Lajčák urged both sides to reach a deal.
But both sides remained entrenched in their positions.
Kosovo’s Prime Minister Albin Kurti said Kosovo would agree to nothing less than "the principle of reciprocity as the basis of any future solution".
Serbia's top Kosovo official Petar Petkovic called Kurti's stance "a premature threat".
Artan Muhaxhiri, a political commentator in Kosovo, said the licence plate talks served as a warning about the lack of political will in both countries to move forward in reaching a final deal on Kosovo's independence.
"The issue of licence plates is a typical example of the complexity of the dialogue between Kosovo and Serbia," said Muhaxhiri. "A topic that really had to be technical has turned into a big deadlock that takes so many months of discussions and dialogues from the Kosovo, Serbian and EU sides to reach an agreement. This means how difficult the final agreement will be if there is no will and commitment of the parties."
Kosovo, a former province of Serbia, declared independence from Belgrade in 2008. Serbia doesn't recognise this and continues to treat Kosovo as if it were under its sovereignty despite a decade of EU-mediated talks between the two sides.