Serbia has recalled its ambassador to Washington in protest at US recognition of Kosovo independence and threatened to withdraw other envoys.
Its Prime Minister, Vojislav Kostunica, told parliament in Belgrade that America had "violated international law for its own interests".
France, the UK, Germany and Italy have also pledged their support for the new state declared on Sunday.
In New York, the UN Security Council is beginning a meeting on the move.
Serbian President Boris Tadic is to ask it to annul the independence declaration, and Belgrade is counting on Russia to veto Kosovo joining the UN as a new nation.
The leading European states which endorsed independence did so after a meeting of EU foreign ministers in Brussels in which it was agreed that Kosovo should not set a precedent for other states.
Spain and several other member-states have withheld recognition because of concerns about international law and separatism.
'First measure'
Mr Kostunica said the recall of Serbia's ambassador to the US was the "first urgent measure of the government which will be implemented in all countries that recognise unilateral independence".
KOSOVO PROFILE
Kosovo Prime Minister Hashim Thaci with the new Kosovo flag
Population about two million
Majority ethnic Albanian; 10% Serb
Under UN control since Nato drove out Serb forces in 1999
2,000-strong EU staff to take over from UN after independence
Nato to stay to provide security
Speaking to Serbian TV from New York, President Tadic said he intended to "demand from [UN Secretary General] Ban Ki-moon the immediate annulment of the independence proclamation by the non-existent state in Kosovo".
Russia's ambassador to the EU, Vladimir Chizhov, told the BBC that Kosovo had little to gain from declaring independence.
"There is no way they will get into the United Nations or the OSCE or the Council of Europe," he said.
"So what will they be getting, changing nameplates at the offices of Western countries in Pristina, calling them embassies?"
Serbia's interior ministry has filed criminal charges against Kosovo Albanian leaders instrumental in proclaiming independence, accusing them of proclaiming a "false state" on Serbian territory.
In Belgrade, about 10,000 students marched in protest at the independence declaration, and Serb enclaves inside Kosovo also saw anti-independence rallies.
Serbian security forces were driven out of Kosovo in 1999 after a Nato bombing campaign aimed at halting the violent repression of ethnic Albanian separatists.
The province has been under UN administration and Nato protection since then.
Pledges of support
On Monday, Washington formally recognised Kosovo as a "sovereign and independent state".
STANCE ON RECOGNITION
For: Germany, Italy, France, UK, Austria, US, Turkey, Albania, Afghanistan
Against: Russia, Spain, Romania, Slovakia, Cyprus
In Brussels, EU foreign ministers adopted a compromise proposal from Spain, one of several countries which argue that Kosovo's independence is a breach of international law and will boost separatists everywhere.
The bloc set aside differences by stressing Kosovo's declaration was not a precedent for separatists elsewhere and pledging that the whole Balkan region would eventually join the bloc.
Unanimous recognition of Kosovo was never at stake at the meeting because the EU has no legal right to recognise new states, BBC European affairs correspondent Oana Lungescu notes.
The question was whether, despite their differences on recognition, Europeans could unite on how to bring stability in their backyard, after almost two decades of seemingly endless Balkan crises.
This is an ugly victory for demographic warfare
It took hours of tortuous negotiations but the EU managed to pass the unity test, our correspondent says.
Kosovo, the ministers agreed, was a unique case and did not call into question international legal principles, such as territorial integrity.
The bloc's statement said the EU was ready to play a leading role in the Balkans, with a 2,000-strong police and justice mission headed to Kosovo and new measures to promote economic and political development in the region, including a donors' conference by June.
The EU foreign affairs chief Javier Solana said there was a total commitment to bring all the Balkan countries into the EU.
But Kosovo will not be able to get very close until it is recognised by all 27 members, and that may take a long time, our correspondent adds.
Among other countries to recognise Kosovo was Turkey.
Correspondents say this has symbolic significance because for centuries the Ottoman Turks ruled the Balkans, including modern-day Serbia and Kosovo.