PARIS -- Kosovo's tennis federation has accused Novak Djokovic of aggravating an already tense situation after he wrote that Kosovo was "the heart of Serbia" on a camera lens after his first-round victory Monday at the French Open.

Thirty NATO peacekeeping soldiers were injured Monday in clashes with Serb protesters in the northern Kosovo town of Zvecan, where Djokovic's father grew up.

Serbs, who comprise a majority in Kosovo's north, have never accepted the country's 2008 declaration of independence from Serbia. They still see Belgrade as their capital more than two decades after a Kosovo Albanian uprising against repressive Serbian rule. Ethnic Albanians make up more than 90% of the population of Kosovo as a whole.

Monday's clashes came as ethnic Albanian mayors took office in Serb-majority areas following elections that the Serbs had boycotted.

"Kosovo is the heart of Serbia. Stop the violence," Djokovic, a 22-time Grand Slam champion, wrote in Serbian on a TV camera at Roland Garros.

Djokovic later explained that he was against war, but he defended his statement and described Kosovo's situation as a "precedent."

"As a son of a man born in Kosovo, I feel the need to give my support to our people and to entire Serbia," he told reporters. "My stance is clear: I am against wars, violence and any kind of conflict, as I've always stated publicly. I empathize with all people, but the situation with Kosovo is a precedent in international law."

Kosovo tennis federation president Jeton Hadergjonaj said in a statement: "The comments made by Novak Djokovic at the end of his Roland Garros match against Aleksandar Kovacevic, his statements at the post-match press conference and his Instagram post are regrettable."

Hadergjonaj accused Djokovic of using his status as a well-known personality to stir tensions.

"Novak Djokovic was already the author of similar actions in the past. Despite a general message against violence, the statement 'Kosovo is the heart of Serbia' and further statements after the match, made by such a public figure, on the occasion of a worldwide event like the French Open, directly result in raising the level of tension between the two states, Serbia and Kosovo," the statement added.

The French Tennis Federation (FFT), which organizes the French Open, said in a statement: "The same rules apply to all four Grand Slams. The tournament referee and Grand Slam Supervisors ensure that these rules are complied with.

"Messages are passed on to the teams of any players concerned by such matters."

A source with knowledge of the matter told Reuters the FFT did not see Djokovic's action as "detrimental to the best interests of the tournament," as per the Grand Slam rulebook.

Neither the International Tennis Federation (ITF) nor the ATP, which manages men's professional tennis, were available for comment on Tuesday.