Serbia's president says he will never accept the independence of Kosovo, after the publication of a UN plan which could allow it to separate.
Mr Tadic said the plan would set a dangerous political precedent
UN Special Envoy Martti Ahtisaari's plan recommends that Kosovo should govern itself democratically and be able to make international agreements.
But President Boris Tadic said the plan paved the way for independence, which he and Serbia would not accept.
Kosovo's leader said he believed the process would end in full independence.
"Kosovo will be sovereign like all other countries," President Fatmir Sejdiu said, after meeting Mr Ahtisaari.
The UN has administered Kosovo since a Nato bombing campaign forced out Serbian troops in 1999.
Talks to determine the province's final status have been continuing for years without the two sides coming to agreement.
KEY PROPOSAL POINTS
Contains no reference to Serbian sovereignty or independence for Kosovo
Blocks Kosovo from joining Albania, or having its Serb areas split off and join Serbia
Kosovo can use national symbols
Kosovo can join international organisations
Creates international envoy mandated by UN and EU with power to intervene in government
Retains Nato and EU forces in military and policing roles
Protects non-Albanian minority with guaranteed roles in government, police and civil service
Protects Serbian Orthodox Church sites and Serbian language
Ethnic Albanians make up 90% of the province's two million people.
According to the UN, more than 220,000 non-Albanian Kosovans are living as internally displaced refugees in Serbia and Montenegro.
The ethnic Albanian majority overwhelmingly want to break away from Serbia, but Serbs regard the province - which is still officially part of Serbia - as the cradle of their culture, and oppose any solution that would lead to its independence.
Mr Ahtisaari says the proposals are a compromise between ethnic Albanian aspirations for an independent Kosovo and Serbia's wish to keep the province as part of its territory.
"I hope that as a result of this process we can make a new beginning. I think it will take some time and I think we have to be realistic as well that to talk about Kosovo as a model multi-ethnic society will take a long time, but it is important that we make a beginning," he said.
But speaking after talks with Mr Ahtisaari in Belgrade, Mr Tadic said that the plan in effect paved the way for Kosovo to become independent.
The proposal "does not explicitly mention independence for Kosovo, but it also does not mention territorial integrity of Serbia," he said. "That fact alone, as well as some other provisions, opens the possibility for Kosovo's independence."
And he insisted that this was not something Belgrade was prepared to accept.
"Imposing independence would violate the fundamental principles of international law and serve as a dangerous political and legal precedent," Mr Tadic said.
Under Mr Ahtisaari's plan, Kosovo would be allowed its own national symbols, including a flag and anthem, and to apply for membership of international organisations like the United Nations.
It would not be unconditional independence, however.
An "international community representative" would be appointed, with powers to intervene if Kosovo tried to go further than the plan allowed, while Nato and EU forces would remain in military and policing roles.
Kosovo could not be partitioned between Serbian and ethnic Albanian areas, nor would Kosovo be allowed to join any other state - implicitly ruling out the creation of a "greater Albania".
The interests of Kosovo's Serbs, including the Serbian Orthodox Church and the language, would be explicitly protected, and there would be guaranteed Serb representation in parliament, the police and civil service.
The UN Security Council will have the final say on whether to adopt the plan.