Serbian Prime Minister Aleksandar Vučić during an interview in Belgrade | Andrej Isakovic/AFP via Getty Images Goodwill short-lived between Kosovo and Serbia after EU talks EU’s Mogherini rebukes Thaçi as Serbia fumes.

Well, that didn't last long.

After weeks of rising tension between Kosovo and Serbia, the leaders of both sides agreed to calm things down Tuesday night at a Brussels meeting with EU foreign policy chief Federica Mogherini.

But comments by Kosovo President Hashim Thaçi after the meeting left Serbian Prime Minister Aleksandar Vučić fuming Wednesday morning — and even prompted a diplomatic but nonetheless clear reprimand from Mogherini.

"An essential outcome was to leave tensions behind and to refrain from commenting in public on the discussions," her spokeswoman Maja Kocijancic said in a written statement. "We regret that this has not been complied with by all, and invite [everyone] to exercise restraint and build on the agreement reached yesterday."

The almost instant resumption of hostile rhetoric between Kosovo and Serbia underlined how fragile relations remain between the two sides, who were enemies in the last of the wars that tore Yugoslavia apart in the 1990s.

Tension spiked earlier this month after Serbia sent a train into Kosovo emblazoned with the slogan "Kosovo is Serbia" in large letters and multiple languages. Serbian officials halted the train inside Serbia after Kosovo deployed special police to the border.

"They don’t do anything in accordance with the things that we agreed upon" -- Aleksandar Vučić

Kosovo, whose ethnic Albanian majority endured years of repression under the rule of Serbian nationalist Slobodan Milošević, unilaterally declared independence from Serbia in 2008. But Belgrade regards Kosovo, which has a central place in Serbian history, as as a rebellious province and has vowed never to recognize it as an independent state.

Alarmed by the rapid deterioration in relations, Mogherini called the presidents and prime ministers of Kosovo and Serbia to Brussels with the aim of getting an EU-sponsored dialogue back on track. Things initially seemed to go according to plan, with both Thaçi and Vučić saying the meeting had helped to reduce tension.

But Thaçi also declared that he thought Serbian leaders had "started to recognize the independence of Kosovo" and that Kosovo had answered "calls for war from Serbia with calls for peace." Those remarks infuriated Vučić as both sides had agreed not to make inflammatory statements.

"It means that they are not able or willing to keep their word for more than two minutes," an angry-sounding Vučić told POLITICO in a telephone interview Wednesday. "They don’t do anything in accordance with the things that we agreed upon."

The rise in tension takes place against the backdrop of upcoming Serbian presidential elections, due in April. Vučić, the dominant figure in Serbian politics, declined to comment on speculation that he may run in that election.

The flare-up over the talks does not seem fatal to the EU-backed dialogue. Vučić said he would have to consult with colleagues before deciding whether to continue with the process. But he is likely to have been mollified, at least to an extent, by Mogherini's indirect criticism of Thaçi.

“I saw yesterday respect and a constructive approach around the table,” Mogherini told reporters Wednesday. “The more they comment on each other, the less this constructive sense of engaging in the dialogue becomes sustainable."

However, the spat does illustrate how hard it has become to make meaningful progress through the six-year-old dialogue, which aims to normalize relations between Pristina and Belgrade. Politicians and commentators in both Kosovo and Serbia have questioned whether their governments should continue with the process. But getting relations between the two sides on a solid footing is essential to both countries' aim of joining the European Union, even if that prospect is many years away.