Serbian world number three Novak Djokovic risked stirring up a political controversy at the French Open on Monday after writing a message about Kosovo following his first-round victory.

“Kosovo is the heart of Serbia. Stop the violence,” 22-time Grand Slam champion Djokovic wrote on a camera lens in Serbian.

Around 25 NATO peacekeeping soldiers defending three town halls in northern Kosovo were injured in clashes with Serb protesters on Monday, while Serbia’s president put the army on the highest level of combat alert.

The incidents came a day after NATO chief Jens Stoltenberg called on Kosovo to tone down tensions with Serbia following clashes between Kosovan police and protesters who are opposed to Albanian mayors taking office in ethnic Serbian areas.

Serbian President Aleksandar Vucic said that 52 Serbs were injured on Monday, three of them seriously, while Kosovo President Vjosa Osmani accused his Serbian counterpart of destabilising Kosovo.

Serbia and its traditional ally Russia do not recognise Kosovo’s independence, and Moscow has blocked the country’s bid to become a member of the United Nations. Serbia still considers Kosovo part of its territory.

“Kosovo is our cradle, our stronghold, centre of the most important things for our country… There are many reasons why I wrote that on the camera,” RFI radio quoted the 36-year-old Djokovic as saying to Serbian media. The French tennis federation (FFT), which organises the event, told Reuters that there were “no official Grand Slam rules on what players can or cannot say. The FFT will not be making any statement or taking any stance on this matter.”

Djokovic will hope to avoid another political distraction at a Grand Slam after he defended his father at the Australian Open in January when a video emerged showing him posing with some fans holding Russian flags amid the war in Ukraine.