A motorway bridge collapsed on Tuesday over the northern Italian port city of Genoa in what the transport minister said was likely to be "an immense tragedy".

The head of the local ambulance service said there were "dozens of dead", Italian news agency Adnkronos reported.

Ten people have been killed based on an initial estimate when a 650 foot section of the Morandi Bridge  came down on train lines in an industrial district of the north-west port city, police sources said on Tuesday.

The sources said 20 vehicles were involved, adding train services had been interrupted.

The local fire brigade said also victims were likely after the bridge collapsed at around 11:30 a.m. (0930 GMT) during torrential rainfall.

Italian television showed images of the collapsed bridge, which was built on the A10 toll motorway in the 1960s. Restructuring work on the bridge was carried out in 2016.

An 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".

Genoa Bridge Collapse The collapsed Morandi Bridge is seen in the Italian port city of Genoa August 14, 2018. Photo: REUTERS/Stringer

Genoa Bridge Collapse A motorway bridge which collapsed on Tuesday near the northern Italian port city of Genoa is seen in this picture released by Italian firefighters on August 14, 2018. Photo: Italian Firefighters Press Office/Handout

Video footage posted by La Repubblica showed the bridge mid-collapse, with parts of the structure slowly tumbling out of sight as fog enveloped the scene. Onlookers could be heard screaming, "Oh my God! Oh my God!