AJACCIO, Corsica (AP) _ Two bombs exploded Thursday on this French Mediterranean island, minutes after a warning from an anonymous caller that four bombs would go off.

Thirteen people were slightly injured in the blasts at the Social Security offices on the outskirts of Ajaccio and the headquarters of the Transport Department in the heart of the Corsican capital, according to the Interior Ministry in Paris.

Both buildings had been evacuated ahead of the blasts, sparing serious injuries.

The twin attacks boosted tensions in Corsica, and in Paris. A nearly quarter-century campaign by separatists, compounded by infighting among the various groups, has stymied authorities.

``This attack was meant to kill,'' said the state's highest representative in Corsica, Prefect Jean-Pierre Lacroix.

Bomb blasts are not unusual in Corsica, but bombings usually happen overnight, sparing injuries.

An anonymous telephone call received at 11 a.m. by Radio France threatened four blasts Thursday _ at the Social Security offices, the Transport Department, the Treasury and the tax offices.

Two bombings followed within 30 minutes.

A false alert later forced the evacuation of the prefecture, or main government building, of Ajaccio.

The blast at the Social Security offices caused serious damage to five floors of the building. Nearby, cars were blown from their parking spots. There was minor damage to the Transport offices.

There was no immediate claim of responsibility for the bombings, but it was widely assumed that they were a continuation of Corsican militants' campaign for greater autonomy.

French President Jacques Chirac and Prime Minister Lionel Jospin, at a summit in London, condemned the bombings.

``This latest step of violence and terrorism in Corsica must drive state and Corsican leaders to react as strongly as possible to this downward spiral of insanity,'' Chirac said at a press conference.

An official from the anti-terrorism division of the state prosecutor's office in Paris was heading to Corsica for the investigation.

Also Thursday, a Paris court sentenced Corsican nationalist leader Charles Santoni to 28 years in prison for the 1996 killing of a member of France's special rapid intervention force, known as RAID.