Office for Kosovo and Metohija Director Marko Djuric said Serbia reacted after Kosovo's special police moved into an area around the Kosovo side of Gazivode Lake. Kosovar President Hashim Thaci visited the area on Saturday.
There was no immediate reaction from Kosovo. Serbia does not recognize Kosovo's 2008 declaration of independence but the two sides have been told they must normalize relations as a precondition to European Union membership.
A NATO-led peacekeeping force has been deployed in Kosovo since the military alliance intervened in 1999 to stop Serbia's onslaught against ethnic Albanian separatists.
Thousands of people in Kosovo are protesting their president's willingness to include a possible territory swap with Serbia in the ongoing negotiations to normalize relations between the two countries.
Supporters of Kosovo's opposition Self-Determination Party held banners and national Albanian flags as they marched through the capital city of Pristina toward Skanderbeg Square.
Kosovo President Hashim Thaci has mentioned a "border correction" that would bring southern Serbia's Albanian-dominated Presevo Valley into Kosovo.
Some Kosovo and Serbian officials have discussed a deal based on where ethnic minorities are concentrated, such as the Presevo Valley and the northern region in Kosovo where many Serbs live.
Kosovo declared independence in 2008 from Serbia, which does not recognize it as a country.
Both countries hope the European Union-facilitated talks will result in a legally binding agreement.