Tadic said Serbia will ask the U.N. Security Council to seek an opinion from the Hague-based court on the legality of Kosovo's declaration of statehood, expected early in 2008.
"We must do everything to prevent Kosovo's independence ... in a peaceful and well-planned way," Tadic told state TV late Monday. "But, if that decision is made, we must annul such a unilateral act by starting legal processes in front of international institutions."
The international court is the U.N.'s highest court for resolving international disputes.
In apparent coordinated action with ally Serbia, Russia's envoy to unsuccessful talks on Kosovo's future status late Monday said the independence declaration would be illegal.
"A unilateral declaration of independence would violate U.N. Security Council resolution 1244" that says Kosovo is a Serbian province, Russia's RIA-Novosti news agency quoted Alexander Botsan-Kharchenko as saying.
"In that case, Russia would demand the reversal of such a decision, its annulment," the agency quoted him as saying.
European Union nations, at a meeting in Brussels on Monday, failed to break a deadlock on whether to recognize the independence of Kosovo - which is still nominally part of Serbia. European nations see unity over Kosovo's status as vital for the 27-nation bloc, and feel that failure to speak with one voice would damage EU credibility in the Balkans.
Kosovo has been run by the United Nations, backed by NATO troops, since the alliance's 1999 bombing campaign to end a Serb crackdown on ethnic Albanian separatists.
Washington has repeatedly signaled that it is ready to recognize Kosovo's split from Serbia, raising the likelihood of a showdown with Russia, a close Serbian ally, when the Security Council takes up the issue on Dec. 19.
Serbia has launched a diplomatic offensive to persuade council members that "Kosovo's independence is absolutely unacceptable," Tadic said.
Serbia has rejected phased-in, supervised statehood for Kosovo, while Kosovo Albanians, who represent 90 percent of the province's 2 million people, want nothing but independence.
On the second day of the St. Petersburg International Economic Forum, a plenary meeting was held, in which Russian President Vladimir Putin, French President Emmanuel Macron, Japanese Prime Minister Shinzo Abe, Chinese Vice President Wang Qishan and IMF head Christine Lagarde took part