BBC south-east Europe analyst
The Mitrovica violence is the worst inter-ethnic clash in four years
Wednesday's outbreak of violence in Mitrovica is the worst inter-ethnic clash in four years - and it marks the first time in two years that crowds have fought each other.
Over the past few years there has been a steady improvement in the security situation in Kosovo.
Kosovo came under UN administration after Nato's campaign of air raids led to the withdrawal of Serb security forces from the region in 1999.
There have been isolated acts of inter-ethnic violence - mostly the shooting of Serbs who remained in Kosovo after tens of thousands of fellow Serbs fled the province five years ago.
But the UN may have believed that the kind of clashes that took place on Wednesday had been consigned to the past.
Now international organisations and countries with a stake in the Kosovo peace process have been quick to condemn the violence in Kosovo.
"These criminal acts are completely unacceptable and... threaten the progress of Kosovo towards a better future," said a spokesperson for the EU foreign policy chief, Javier Solana.
"The EU wants the perpetrators to be brought to justice. The EU appeals to all political leaders to join in the condemnation and in the fight against violence."
Next Wednesday marks the fifth anniversary of the launch of Nato's air strikes - and the period around the anniversary is usually a tense time.
It reminds Albanians of Belgrade's campaign of ethnic cleansing against them. Serbs recall how they became the targets of violence and expulsion after the Serb security forces were withdrawn from Kosovo.
Hardline ethnic Albanians opposed to talks with Belgrade may also have inspired some of the violence which followed the opening of talks earlier this month between Albanian and Serbs officials.
The talks were the first substantive negotiations between the two sides since the war.
Others have been enraged by the suggestion of the new Serbian Prime Minister, Vojislav Kostunica, that Kosovo should be divided into cantons along ethnic lines. The Albanians reject this policy on the grounds that it would amount to Kosovo's partition.
The violence is also taking place against the background of the run-up to Kosovo's parliamentary elections, due in the autumn.
Political polarisation is part and parcel of the, as yet, unofficial election campaign - and not all of it is directed against the other side on the ethnic divide.
In the most notable incident of its kind, last week a grenade was thrown at the residence of Kosovo's President, Ibrahim Rugova. Mr Rugova represents the more moderate wing among Kosovo's ethnic Albanian politicians.
That attack is very likely to have been the work of Kosovar Albanian extremists.
But, for now, the real danger is the flare-up in inter-ethnic violence.
As the UN administration works on establishing standards of locally-run democratic governance and the rule of law, the last thing it wants is a return to the instability that was a hallmark of its first few years in Kosovo.