The collapsed Morandi Bridge is seen in the Italian port city of Genoa. Picture: Stringer/Reuters

Rome - A motorway bridge collapsed on Tuesday over the northern Italian port city of Genoa, killing dozens of people according to the local ambulance service, in what the transport minister said was likely to be "an immense tragedy".

The local fire brigade said the bridge collapsed at around 11.30am (0930 GMT) during torrential rainfall. Television showed images of the collapsed section of the bridge, built on the A10 toll motorway in the 1960s.

The head of the ambulance service said there were "dozens of dead", according to Italian news agency Adnkronos. At least 10 people were killed and 20 vehicles were involved, local police sources said.

An ambulance official told Reuters the service could only confirm two injured people so far, "but we suppose there are unfortunately a lot of dead".

Restructuring work on the bridge was carried out in 2016. The highway operator said work to shore up the foundation of the bridge was being carried out at the time of the collapse, adding that the bridge was constantly monitored.

The elevated road crumbled over a river, railroad tracks and buildings. The highway is a major artery to the Italian Riviera and to France's southern coast. Train services around Genoa have been halted.

A witness told Sky Italia television he saw "eight or nine" vehicles on the bridge when it collapsed in what he said was an "apocalyptic scene".

Transport Minister Danilo Toninelli said in a tweet that he was "following with great apprehension what seems like an immense tragedy".

Shares in Atlantia, the toll road operator which runs the motorway, were suspended after falling 6.3 percent after news of the collapse.