Why tensions have boiled over in Kosovo and why Novak Djokovic weighed in

Serbian tennis legend Novak Djokovic stirred up political controversy at the French Open on Monday, after writing a message about Kosovo on a camera lens following his first-round victory.

"Kosovo is the heart of Serbia. Stop the violence," Djokovic wrote in Serbian.

Here's more on the rising tensions between Serbia and Kosovo that Djokovic was referring to.

Dozens of NATO troops secured a municipal building in the Kosovo town of Zvečan on Tuesday, after 30 NATO soldiers and 52 Serb protesters were injured in clashes the previous day.

NATO retains 3,700 peacekeeping troops in Kosovo, the remainder of an original 50,000-strong force deployed in 1999. NATO said Tuesday 700 more are on their way.

Police said in a statement that the situation is "fragile, but calm," though Serbian protesters smashed two cars belonging to journalists in the Kosovar town of Leposavic on Tuesday.

The situation is fuelling fears of a renewal of the 1998-99 conflict in Kosovo, which claimed more than 10,000 lives and left more than one million homeless.

Kosovo President Vjosa Osmani said criminal gangs, supported by Serbian President Aleksandar Vucic, aim to destabilize Kosovo and the entire region. Vucic blames Kosovo authorities for causing problems by installing new mayors despite the non-participation of the masses.

The dispute over Kosovo is centuries-old. Serbia cherishes the region as the heart of its statehood, pointing to a 1389 battle with Ottoman Turks. Also, numerous medieval Serb Orthodox Christian monasteries are in Kosovo. 

Kosovo's majority ethnic Albanians view Kosovo as their country and accuse Serbia of occupation and repression. Ethnic Albanian fighters launched a rebellion in 1998 to end Serbian rule. Belgrade's brutal response prompted NATO to intervene, and Serbia withdrew, with international peacekeepers deployed.

Kosovo declared its independence in 2008, recognized by well over 100 countries, but not Russia or China.

Ethnic Albanians make up more than 90 per cent of the Kosovo population. Ethnic Serbs account for just five per cent of its 1.8 million people, but are heavily concentrated in the north near Serbia and have long demanded the implementation of an EU-brokered 2013 deal for the creation of an association of autonomous municipalities in their area.

Some 50,000 minority Serbs refuse to recognize Kosovar state institutions, receive their pay and benefits from Serbia's budget and pay no taxes either to Pristina or Belgrade.

Tensions had been ticking upward for months in a dispute that began over car licence plates.