Clashes were reported on Wednesday from Mitrovica in the north to Urosevac in the south and Pec in the west as NATO peacekeeping troops scrambled to quell the outbreak.
"There are 14 people killed, six in Mitrovica, three in Lipljan, three in Caglavica, one in Pec and one in Urosevac," Pleurat Sejdiu, of the Kosovo Health Ministry, said. One person died in Pristina, the capital.
The Serbian news agency Beta quoted local police as saying Albanian mobs in Lipljan were burning Serb houses as NATO soldiers worked to evacuate Serb civilians.
Kosovo has been under the control of the United Nations since NATO bombing drove Serb forces out in mid-1999, halting Serb repression of Albanian civilians but also granting victory to Albanian separatist fighters.
United Nations Kosovo police veteran Derek Chappell described it as "a very dangerous situation ... very large scale."
In the late evening, hundreds of angry Albanians surrounded a Serb enclave in Pristina, setting UN vehicles on fire and stoning Kosovo police who responded by firing rubber bullets.
"A very dangerous situation ... very large scale"
UN Kosovo police veteran
The violence began when Albanians massed in Mitrovica to vent their rage at Tuesday's drowning of two boys.
The two children's bodies were found after dozens of soldiers, police and civil emergency workers searched the Ibar river near the village of Cabra, about 40km north of the province's capital Pristina, said Angela Joseph, a UN police spokeswoman.
The search was launched after reports that three ethnic Albanian children had disappeared in the swirling waters on Tuesday afternoon. A third child was still missing.
Fitim Veseli, a 13-year-old who said he was with the missing children, said they had been chased into the river by Serbs from a neighbouring village. Veseli's nine-year-old brother, Florent, was among the missing.
The drownings occurred a day after a 19-year-old Serb was shot and wounded in central Kosovo.
The shooting provoked a protest on Tuesday by angry Kosovo Serbs, who blocked a key road linking the province's capital with neighbouring Macedonia.
UN Secretary-General Kofi Annan called for an end to the violence, which he said jeopardised the stability of Kosovo and the US State Department warned it could wreck the peace.
In Belgrade, Serbia and Montenegro's Supreme Defence Council met and an army source said raising combat readiness and a possible movement of troops were discussed.
But Prime Minister Vojislav Kostunica earlier ruled out any military response inside Kosovo - a move which would trigger a major confrontation with NATO.