Three of Serbia's neighbours have announced they are to recognise Kosovo's independence.
Croatia and Hungary - two of Serbia's neighbours - have officially recognised Kosovo's independence.
Bulgaria, Croatia and Hungary said their decision was "based on thorough consideration" and EU's conclusions.
A similar move is expected on Thursday by Bulgaria, another Balkan nation.
Serbia's foreign minister said Kosovo was "an illegal state", warning that nations that had recognised it "cannot count on good relations with us".
Serbia's foreign minister said Kosovo was "an illegal state", warning that nations that had recognised it "cannot count on good relations with us".
Separately, UN police began returning to the Kosovo Serb part of Mitrovica after bloody clashes there on Monday.
Separately, UN police began returning to the Serb part of the Kosovo town of Mitrovica, after clashes there in which a UN officer was killed on Monday.
The police left the northern part of the town after fighting pitched battles with Kosovo Serbs who had earlier seized a city courthouse.
The police left the northern part of the town after fighting pitched battles with Kosovo Serbs who had earlier seized a city courthouse.
A UN policeman from Ukraine was fatally wounded during the clashes, and more than 130 people - both protesters and international forces - were wounded.
A UN policeman from Ukraine was fatally wounded during the clashes, and more than 130 people - both protesters and international forces - were wounded.
Belgrade's warning
'Difficult to swallow'
Croatia, Bulgaria and Hungary announced their intention to recognise the authorities in Pristina in a joint statement on Wednesday.
On Wednesday, Croatia and Hungary officially announced that they had recognised Kosovo as an independent state.
Croatia and Hungary later officially confirmed they had recognised Kosovo and a further announcement was expected from Bulgaria on Thursday.
"The government has accepted the decision of the Kosovo parliament on declaring the independence of Kosovo on 17 February," a statement by the government in Zagreb said.
The foreign ministry in Budapest said that, in common with the majority of EU and Nato member states, it had decided that "internationally supervised independence may offer the best way out of the crisis".
Croatian Prime Minister Ivo Sanader told reporters he realised that the decision "is a difficult one for Serbia to swallow".
"But I don't expect a worsening of political and economic relations because there is no alternative to good neighbourly relations," Mr Sanader said.
Croatia's Deputy Prime Minister Slobodan Uzelac, an ethnic Serb, immediately tendered his resignation in protest against the move.
Croatia is the second former Yugoslav republic to recognise Kosovo, after Slovenia.
Croatia is the second former Yugoslav republic to recognise Kosovo, after Slovenia.
On Wednesday, Serbian Foreign Minister Vuk Jeremic urged Serbia's neighbours not to recognise Kosovo, warning that there would be consequences.
In Budapest, the foreign ministry said that, in common with the majority of EU and Nato member states, Hungary had decided that "internationally supervised independence may offer the best way out of the crisis".
The Bulgarian foreign ministry said Sofia would officially recognise Kosovo on Thursday.
The announcements came shortly after Bulgaria, Croatia and Hungary said in a joint statement that they intended to recognise the authorities in Pristina.
The statement described Kosovo's case as unique and also made it clear that Bulgaria, Croatia and Hungary would support EU and Nato efforts to build democracy in Kosovo.
The document also expressed hopes that Belgrade would keep "its European orientation".
Hungary and Bulgaria are both part of the 27-member EU, and Croatia is negotiating EU membership.
Belgrade's warning
On Wednesday, Serbian Foreign Minister Vuk Jeremic urged Belgrade's neighbours not to recognise Kosovo, warning that there would be consequences.
"I call on states, particularly those of the region, not to take this step. Do not injure our country's territorial integrity and sovereignty," he said.
"I call on states, particularly those of the region, not to take this step. Do not injure our country's territorial integrity and sovereignty," he said.
Serbian President Boris Tadic had already warned Zagreb that recognising Kosovo would have an immediate negative impact on relations.
Serbian President Boris Tadic had already warned Croatia that recognising Kosovo would have an immediate negative impact on relations.
Croatia's own declaration of independence in 1991 prompted a four-year conflict with Serb-led Yugoslavia.
Croatia's own declaration of independence in 1991 prompted a four-year conflict with Serb-led Yugoslavia.
Ties have steadily improved but Belgrade is likely to take diplomatic action.
Ties have steadily improved but Belgrade is likely to take diplomatic action.
Serbia has recalled its ambassadors from about 30 countries that have recognised Kosovo.
Serbia has recalled its ambassadors from about 30 countries that have recognised Kosovo.
'European orientation'
The joint statement describes Kosovo's case as unique.
It also makes it clear that Bulgaria, Croatia and Hungary will support EU and Nato efforts to maintain stability and build democracy in Kosovo.
Significantly, they base their decision on the judgements of EU leaders and foreign ministers and they express the hope that Belgrade will keep "its European orientation".
Hungary and Bulgaria are both part of the 27-member EU, and Croatia is negotiating EU membership.
While most EU member states have already recognised Kosovo's independence, a significant minority - including Spain, Slovakia, Romania and Cyprus - have refused to accept the declaration.
While most EU member states have already recognised Kosovo's independence, a significant minority - including Spain, Slovakia, Romania and Cyprus - have refused to accept the declaration.
Are you in Bulgaria, Croatia or Hungary? Do you agree with your government's decision to recognise Kosovo? Or are you in Serbia or Kosovo? How significant a development is this? Send us your comments using the form below.
Are you in Bulgaria, Croatia or Hungary? Do you agree with your government's decision to recognise Kosovo? Or are you in Serbia or Kosovo? How significant a development is this? Send us your comments using the form below.
