BELGRADE – EU High Representative for Foreign Affairs and Security Policy Federica Mogherini voiced concern on Monday over the escalation of tensions between Serbia and Kosovo, following the incident with the Serbian passenger train, and called on the parties to engage in dialogue.

On Saturday, the first passenger train since 1999 heading from the Serbian capital of Belgrade to Kosovo was stopped near the administrative border over security concerns.

The train was colored as the Serbian flag with the caption “Kosovo is Serbia” written in 21 languages on it.

“I was personally not only following the issue of the train over the weekend, but also personally… I was in contact with Prime Minister Vucic. Extremely worried of situation that could have easily escalated. And my message to the parties has been to avoid escalations, try to contain both acts and rhetoric, and try to see at the common engagement through dialogue as something that is delivering for both,” Mogherini told reporters after a meeting of EU member countries’ foreign ministers.

According to Mogherini, the progress made over the past year by the European Union with both Belgrade and Pristina is “historical.”

She noted that “tensions are still high,” but praised the leadership of the two countries for their work and determination.

The Kosovar authorities are opposing railway communication between Serbia and Mitrovice saying that it is a threat to Kosovo’s sovereignty.

On January 15, Serbian Prime Minister Aleksandar Vucic sounded like a leader whose country had just averted mortal danger.

“If I had not asked the Serbs to stop the train, we would have had war,” Vucic told Belgrade’s Pink TV.

Kosovo President Hashim Thaci had ordered a Rosu special-police unit to halt the train, prompting local Serbs in Mitrovica to come out to protest his move.

Serbian President Tomislav Nikolic emerged from a January 15 meeting of his council for national security saying, “We don’t want war, but if it is necessary to protect Serbs from being killed, we will send an army to Kosovo. We will send soldiers; we’ll all go. I’ll go, and it won’t be the first time that I go [to defend Serbs]. Serbia will act in line with the Serbian Constitution.”

Nikolic was critical of the outgoing U.S. administration over its support for Kosovo.

“I think these are the lasts gasps of the current U.S. administration, whose members have had streets [in Kosovo] named after them,” Nikolic said, adding that “neither the EU nor NATO reacted as they should have to [the January 14] events.”

NATO said on Monday it was watching closely the situation between Belgrade and Pristina, and urged all parties to exercise restraint and continue the dialogue, Radio Free Europe reported.

The statement was released from the NATO headquarters as a comment on the latest developments in the relations between Belgrade and Pristina.

Serbia does not recognize Kosovo as an independent state, which was unilaterally proclaimed in 2008.