The blast occurred in Prizren, some 50 miles southwest of the province's capital, Pristina, U.N. spokesman Ken Stica said.

Police believe the explosion was caused by an improvised explosive device under the vehicle, said Fatmir Gjurgjeali, a police spokesman in the city of Prizren.

The top U.N. official in Kosovo, Soren Jessen-Petersen said the mission was "shocked."

Kosovo formally remains part of Serbia-Montenegro, the union that replaced Yugoslavia, but has been a U.N. protectorate for more than five years.

Its final status remains unresolved, with the ethnic Albanian majority pushing for independence and Serbs intent on remaining part of Serbia-Montenegro.

Some 3,500 international police officers, including about 50 Nigerians, serve in the U.N. force that was deployed in Kosovo in 1999 when the United Nations started to administer the province.