Ahtisaari, a former president of Finland, has been tasked with mediating planned direct negotiations between Serbs and Kosovo's majority ethnic Albanians. The UN Security Council backed the start of the negotiations in October.
Ethnic Albanians, who make up 90 percent of Kosovo's population, seek the territory's independence, but this is opposed by Serbs who consider the land to be the cradle of their statehood and religion. Serb leaders have proposed broad autonomy for the province.
Serbia lost authority over Kosovo after NATO's 1999 bombing campaign ended a crackdown by Slobodan Milosevic's forces against ethnic Albanian rebels.
Kosovo has been under UN administration since then.