A powerful cyclone has killed at least 17 people on the Italian island of Sardinia.

Prime Minister Enrico Letta described the devastation brought by Cyclone Cleopatra as a "national tragedy" and called an extraordinary cabinet meeting to declare a state of emergency.

"Events like this are recorded once every thousand years and their strength goes way beyond any reasonable forecast," said Sardinia Governor Ugo Cappellacci. "Unfortunately the situation is tragic."

Torrential rains caused flooding and landslides that downed bridges and swept cars away.

Authorities said six people remained unaccounted for and warned the death toll might rise as rescuers were reaching isolated areas that were swamped by flood waters which sparked power cuts and damaged phone lines.

The northeastern city of Olbia, the province capital in the touristic Costa Smeralda area, appeared to be the worst affected by the storm.

Italy's civil protection chief, Franco Gabrielli said more than 400mm of rain fell on the area in less than 24 hours.

Among the victims were an entire family of four who died in the city of Arzachena and a policeman who was opening the way to an ambulance when a bridge collapsed under his police vehicle, turning it into a ravine.

Three other policemen who were inside the car were injured in the crash and have been taken to a nearby hospital.

The government has allocated €20m (£16.7m) to boost relief efforts.