At separate talks with the troika of international mediators on the province's status, Serbians and Kosovar Albanians failed to reach a breakthrough, but agreed to avoid provocative actions while the negotiations are ongoing.
Kostunica said: "Serbian authorities are developing various 'reaction models' in the event of the least favorable solution for the country on the future status of Kosovo, namely a unilateral declaration of independence by the Kosovar Albanian leaders."
Russia's envoy at Thursday's talks, Alexander Botsan-Kharchenko, said "there was no breakthrough - but none of us had expected a breakthrough."
At the talks, Kosovo's negotiators insisted that a plan foreseeing the province's eventual independence drafted by United Nations special envoy Martti Ahtisaari could not be renegotiated.
Kosovo has been a UN protectorate since NATO's 78-day bombing campaign against the former Yugoslavia ended a conflict between Serb forces and Muslim Albanian separatists in 1999. The province has been striving for independence from Serbia ever since.
The Serbian prime minister said all measures had to be taken to prevent Kosovo from proclaiming its independence unilaterally, as such a move would set a dangerous precedent, and seriously undermine the authority of the UN.
However, Kostunica did not specify how Belgrade would react given such a move by Kosovars: "It would be incorrect to speak about this at a time when a new round of talks in Vienna on the Kosovo settlement is underway."
The diplomatic troika made up of envoys from the European Union, the United States and Russia launched a 120-day effort to end the stalemate over Kosovo. The troika has to find a compromise by December 10 between Kosovo's demands for independence and Serbia's rejection of Kosovars' bid for secession, and its offer for "essential autonomy."
The Serbian premier expressed his hope that the UN Security Council would not venture to pass a resolution depriving a UN member state of a major part of its territory. "So far this has not happened, thanks to the political cooperation between Serbia and Russia."
Moscow, a traditional ally of Belgrade, has consistently blocked plans for Kosovo's independence, using its UN Security Council veto.