Serbian President Tomislav Nikolic has accused Kosovo of seeking to provoke "war" after a Serbian train, painted with the slogan "Kosovo Is Serbia," was halted before it entered Kosovo.
Nikolic said on January 15, "Yesterday, we were on the verge of clashes."
Nikolic warned that Serbia will defend "every inch of its territory."
On January 14, Serbia decided to stop the train from traveling from Belgrade to Serb-dominated northern Kosovo amid reports of a planned attack by ethnic Albanians.
The train, which Serbia bought from Russia, is painted in the colors of the Serbian flag and features pictures of churches, monasteries, and medieval towns, as well as the words "Kosovo Is Serbia" in 21 languages.
The trip would have been the first from the Serbian capital to the town of Mitrovica in Kosovo since the 1998-99 war between Yugoslav forces and ethnic Albanian separatists.
EU foreign policy chief Federica Mogherini said in a statement that she and her team have been "closely" following the developments in relations between Belgrade and Pristina over the past days.
"The EU calls on both sides to show the necessary restraint and reason," the statement said, while stressing that issues of mutual concern should be resolved through dialogue.
"The dialogue so far has delivered very tangible, concrete, and positive results for the people on the ground and progress in the normalization of relations between the two sides -- and this should not be lost," the statement added.
Serbia and Kosovo, which declared independence from Belgrade in 2008 and is recognized by more than 100 countries, have been involved in EU-facilitated talks since 2011 to normalize ties.