"I condemn in the strongest terms these unlawful acts. These acts physically hurt police in uniform who were doing their duty," Mr Steiner said. "Politically, they hurt the legitimate interests of Serbs in Kosovo."

The police came under fire and had grenades thrown at them when hundreds of Serbs rioted. Earlier, the police had tried to set up a checkpoint in the Serb-dominated northern part of Mitrovica.

UN police officers responded to the rioters with tear gas and gunfire.

Serb rally

Most of the injured policemen suffered shrapnel wounds.

An American UN policeman who was among the injured is being treated at Camp Bondsteel, the main US base in Kosovo.

A local Serb leader, Marko Jaksic, said that of 12 Serbs injured in the clash, two were in serious condition.

Steiner called for "international solidarity" in Mitrovica

Thousands of Serbs rallied in Mitrovica on Tuesday to press for the release of a hardline leader, Slavoljub Jovic-Pagi, arrested during the disturbances.

Following the clashes, the police were withdrawn from patrolling in the north of the city and retreated to a UN compound.

Many of the local Serbs are strongly opposed to the UN's presence in the region.

It is almost three years since the UN took over the administration of the Yugoslav province and control of Mitrovica.

But still, the mission is struggling to complete operations that might be described as routine police work in other countries.

The latest violence has raised further questions about its ability to maintain law and order.