A bridge has collapsed in Italy's northwestern city of Genoa sending vehicles falling nearly 100 metres to the ground and killing at least 30 people, according to local authorities.

Many others were seriously injured, Italian Interior Minister Matteo Salvinisaid, after part of the Morandi Bridge on theA10 motorway caved in around midday local time (10:00 GMT) on Tuesday.

About 200 firefighters were deployed to the scene, with the death toll expected to rise.

The cause of the disaster was not immediately clear, although weather services in the region had issued a storm warning Tuesday morning.

A director for motorway operator Autostrade, which manages the section of the toll road that collapsed, said it had been impossible to predict.

"The bridge was constantly monitored, even more than was foreseen by the law ... there was no reason to consider the bridge dangerous," Stefano Marigliani told Reuters news agency.

Italy's Civil Protection Agency, meanwhile, said it was yet to establish the cause of the incident due to a lack of sufficient information.

More than 30 cars and about 10 trucks were on the viaduct when it collapsed, according to the agency.

Television images showed the bridge in the mist with a huge chunk missing, with Italian media reporting that about 200 metres of the bridge had fallen away.