A senior European Union official held an urgent meeting with the leaders of Serbia and Kosovo and encouraged them to work harder to normalize their relations following a recent spike in tensions.

EU foreign policy chief Federica Mogherini called on them to put aside their differences during meetings and a dinner in Brussels, according to a statement from her office issued late Tuesday.

"I underlined that progress in the normalization of relations between Kosovo and Serbia is essential: for Kosovo and Serbia, for the European Union itself, and the Western Balkans region as a whole," Mogherini said.

Mogherini described the meeting as "open and very constructive" and said they discussed "the developments over the past days, agreed to leave the tensions behind and to focus on the work ahead."

Her meeting was with Prime Ministers Aleksandar Vucic of Serbia and Isa Mustafa of Kosovo, and Presidents Tomislav Nikolic of Serbia and Hashim Thaci of Kosovo.

Thaci on Wednesday described the meeting as "difficult and grave" adding that he was "pleased the main goal of the meeting, lowering tension and avoiding unnecessary provocation, was achieved."

In an interview with, Thaci said he was "more optimistic developments will go along the proper direction, faster, in a more qualitative way under the leadership of Mrs. Mogherini."

"I believe that peace achieved between Kosovo and Serbia will be a stable one for the whole Western Balkans," he said.

Tensions starting building in December when the Serb minority erected a wall at a bridge in Mitrovica, saying it was a barrier to prevent landslides. But ethnic Albanians and others in the ethnically divided northern town said it was erected to keep them out.

The friction increased in early January with the detention of Ramush Haradinaj, a former Kosovo prime minister, and days later when a Serbian train with signs reading "Kosovo is Serbia" was turned back from the border with Kosovo.

Kosovo declared independence from Serbia in 2008, but that hasn't been recognized by Belgrade.

Serbia, backed by Russia, has sought to maintain influence, especially in Kosovo's north where most of the country's Serb minority lives. NATO-led troops have controlled Kosovo's territory since a three-month air war in 1999 to stop a bloody Serbian crackdown against ethnic Albanian separatists.

For years, Brussels has been facilitating a Serbia-Kosovo dialogue to normalize their ties, coping with confrontational stands in both countries where some political groupings have called to end it.

Thaci said there are "many things we could cooperate based on the European values and criteria because integration into the EU is the goal of both our countries."

He said the talking was important: "People and states in times of crisis, conflicts, problems, tension, even wars, sit down and discuss."

Mogherini said both sides "agreed to take the dialogue forward in a spirit of respect, cooperation and mutual understanding," intensifying it over the next days with a series of high-level rounds.