(Adds U.N. statement saying victim was police officer, details)

PORT-AU-PRINCE, April 12 (Reuters) - A U.N. peacekeeper from Nigeria was shot and killed in the Haitian capital on Saturday as tensions fueled by riots over food prices continued to simmer in the Caribbean country, officials said.

Witnesses said the uniformed officer was taking food to colleagues at a police station near the National Palace when he was attacked by a group of "bandits" near the Bel-Air slum, not far from the main Catholic cathedral in downtown Port-au-Prince, a police official said.

He was dragged from his car and shot, the official said.

The United Nations said the victim was a 36-year-old member of the Nigerian police squad taking part in the Haiti peacekeeping operation.

"(The U.N. force) will pursue the authors of this abject crime with the greatest determination," the U.N. Stabilization Mission in Haiti, known as MINUSTAH, said in a statement.

Port-au-Prince has been relatively tranquil since President Rene Preval called on protesters on Wednesday to stop rioting over soaring food prices. At least five people died during a week of violence in which U.N. troops battled demonstrators.

Opposition senators on Saturday voted to fire the impoverished country's prime minister over the sky-rocketing cost of food, which has had a painful impact in countries like Haiti, where most people earn less than $2 a day.

A U.N. spokesman was not available for comment.

There are about 9,000 Brazilian-led U.N. peacekeepers and civilian police in Haiti. The force was sent in after President Jean-Bertrand Aristide was ousted in an armed rebellion in 2004.

Thirty-three U.N. personnel, not including the latest fatality, have died during peacekeeping operations in Haiti, according to the MINUSTAH Web site. (Reporting by Joseph Guyler Delva, Writing by Michael Christie)

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