GENOA, Italy (Reuters) - At least one person died when flood waters swept through the northwestern Italian city of Genoa following unexpectedly heavy overnight rain, which caught officials unprepared despite similar deadly floods less than three years ago.

One of Genoa's biggest rivers, the Bisagno, burst its banks shortly before midnight after days of heavy rain, sending torrents of muddy water through the streets of the medieval port city, smashing shop windows and sweeping aside cars.

The mayor of Genoa, Andrea Doria, said authorities had been given no warning by Arpal, the meteorology agency for the northwestern Liguria region, which had forecast heavy rain and storms but not the blast of water which fell overnight.

"We act on the basis of the Arpal forecasts, which gave no signs of alarm," Doria told a news conference. "The alarm only arrived this morning," he said.

Emergency services said they had recovered the body of a 57-year-old man who had apparently been swept away by the deluge. An intercity train to Milan was also reported to have derailed because of a landslide to the north of the city.

Crews clearing debris from the streets early on Friday found cars piled up on top of each other and sunk into huge holes in the roads. Thick layers of mud reached high up many shop walls.

Italy's mountainous geography has always been vulnerable to environmental disasters, but the impact of storm and landslide damage has been exacerbated in recent years by unregulated building and the poor state of some public infrastructure.

Wedged into an isolated pocket between the mountains and the sea, where a mix of cold and warm air frequently leads to heavy rain, Genoa has always been subject to storms and flooding from the steep hillsides behind the city.

Flooding in Nov 2011 killed at least seven in the city.

Italy's national council of geologists, which warned of looming problems in the fragile area around the city in a statement from January, was heavily critical, saying little had been done since the disastrous 2011 floods. "It's a mass of problems together. You have houses built in the wrong places, inadequate water channelling systems, poor planning and administration," Carlo Malgarotto, president of the council in the region of Liguria, told Reuters.

Environment Minister Gian Luca Galletti said in a statement that 35 million euros (S$56 million) had been earmarked to reinforce flood defences around the Bisagno but the funds had been blocked by a legal dispute. "It's painful to have to repeat that these projects have to start immediately, decisively and with top speed and complete transparency," he said.