An apparent attack, the explostion is a high-profile blow to efforts to integrate France's minorities.

The blast occurred on Sunday at about 04:30 (0330 GMT) in Nantes, western France, close to the home of Aissa Dirmoucha, 57, police sources said.

On Wednesday, the French government appointed Dirmoucha prefect of the eastern Jura area, a post that includes regional responsibility for law and order.

"There is no doubt that this concerns a criminal act," said Jean-Marie Huet, state prosecutor in Nantes. "Tests have been carried out on the shell of the vehicle so we can conduct analyses to determine the nature of the explosive used."

Justice Minister Dominique Perben, speaking in Strasbourg, told reporters he was outraged: "Symbols of the Republic and the authority of the state cannot be attacked," he said.

The vehicle was empty at the time of the blast and no one was hurt, the police sources said. Huet said the attack was well prepared.

"This attack... targeted the new prefect personally and the symbol that he represents", he said.

First minority member

Dirmoucha was not the first member of a minority to be nominated for an influential post in France, but his appointment was particularly sensitive at a time when the country is debating how to integrate its Muslim minority better.

The apparent attack underlines the problems in assimilating France five million Muslims and other minorities.

On Saturday, thousands of Muslims protested against a looming ban on Islamic headscarves in French state schools.

Many French politicians and voters support the planned law

The centre-right government hopes the headscarf ban will prevent the far-right National Front from cashing in on racial strife in regional elections in March. Many on the far right resent immigrants taking influential posts.

Dirmoucha symbolises success among France's immigrants. Born in an Algerian village, he later came to France and had been director of the Ecole Superieure de Commerce business school in Nantes since 1989 before being appointed prefect of Jura.