By Alasdair Sandford BBC News, Paris

The arrest of sailors who hijacked a ferry has sparked protests

France's President Jacques Chirac has condemned a rocket attack on a government building in Corsica, which caused some damage but injured no-one.

The attack came as protests continued over plans to privatise a ferry firm.

Strikes at ports and airports threaten to cut off the French island, leaving hundreds of people stranded.

On the French mainland, the country's largest port, Marseille, has remained closed for a fourth day because of strike action.

The rocket was fired on Thursday night at the prefecture, the authorities' headquarters in Corsica's main city of Ajaccio.

The building was damaged but there were no casualties. No-one has claimed responsibility.

Seaports closed

Meanwhile, a dispute over the future of the state-owned ferry company SNCM has seriously escalated, fuelling nationalist discontent and threatening the island with a total blockade.

Corsica's main seaports are closed and strikes by airport staff have left hundreds of passengers stranded, although some flights have continued in and out of Ajaccio.

A strike by lorry drivers is threatening fuel supplies.

Earlier, protesters broke into a regional assembly building demanding the release of four leaders of the pro-nationalist Union of Corsican Workers.

They are being held on the French mainland accused of hijacking an SNCM ferry late on Tuesday.

Helicopters were used to enable troops and Navy commandos to storm the ship, after striking ferry workers seized it in Marseille and set sail for Corsica.