Serbia and Kosovo need to immediately put aside recent bickering, according to the European Union’s diplomatic chief Federica Mogherini. Her warning on December 18 came amid a major flare-up in tensions between Belgrade and Pristina.

Negotiations, led by Brussels, have been unsuccessful. Ties between Serbia and Kosovo have plunged into crisis in recent weeks, most recently by Pristina’s decision to build an army.

As reported by the Agence France-Presse (AFP), Mogherini told a press conference with Serbian Prime Minister Ana Brnabic that the two sides’ hopes of ever joining the EU depended on resolving their differences through talks and warned of the “very dangerous” alternative to serious dialogue.

According to AFP, things went from bad to worse last week when the Pristina parliament voted to transform its small Kosovo Security Force (KSF), into an army with 5,000 troops, enraging Serbia and alarming many Western powers.

After talks in Brussels with EU officials, Brnabic warned the new army was “the single biggest threat to regional peace and stability” and defended Serbia’s right to lobby countries not to recognise Kosovo.

“This shows exactly and perfectly well the choice that is in front of the two sides — I do something, you do something, I respond to this, you respond to that. Fine, good luck. This has never led any region, forget the Balkans, towards something good,” Mogherini said.

Kosovo’s President Hashim Thaci defended the decision to create an army as he headed to a UN Security Council meeting about the move, insisting it was “irreversible” and that he has the backing of the United States.