The campaign of violence has picked up over the summer

Mr Perben flew immediately to the low-security Casabianda prison on the east coast, which was hit by three bombs last week.

He attempted to calm the fears of prison and law enforcement officials about the escalating violence.

"I wanted... to comfort the guards and their families who were extremely shocked by the attack," he said afterwards.

"We are going to take a certain number of immediate security measures."

The bombs caused heavy damage but no casualties.

A decades-old campaign of low-level violence by Corsican separatists has picked up over the summer.

Challenge

The militants appear to have been responding to:

• the conviction in July of eight separatists involved in the 1998 assassination of the island's governor, Claude Erignac
• the arrest of the fugitive Yvan Colonna, accused of pulling the trigger in the same killing
• a referendum on greater autonomy, which was narrowly defeated pm 6 July

Mr Perben is the first French minister to visit the island since the referendum.

Apart from the prison bombing last Tuesday, a number of incidents have troubled the island in recent days.

On Thursday last week a courthouse was sprayed with gunfire, and police defused three bombs left at an airport police station.

French newspapers were prompted to describe the troubled island as the biggest challenge facing the government.

Government 'firmness'

Of 200 attacks blamed on separatists this year, 50 occurred in the month of July alone.

There were 127 attacks in the same period last year.

Traditionally militants have left bombs outside official buildings overnight, in order to avoid killing people.

Empty holiday homes have also been targeted - four were destroyed immediately after the July referendum.