He also said he would make an initial €5m ($5.7m) available from central funds to the north-western region.
The regional government had requested the action following Tuesday's disaster, which left 39 people dead.
There has been anger and disbelief in Italy that such a vital structure could have simply given way.
Rescuers have said there is little hope of finding more survivors underneath the Morandi bridge, where dozens of vehicles fell from a height of 45m (148ft) in Tuesday morning's collapse.
The cause is not yet known but there has been heavy criticism of the private company responsible for operating the bridge.
What is the latest at the scene?
Hundreds of firefighters worked overnight and throughout Wednesday with lifting gear, climbing equipment and sniffer dogs to try to locate survivors and bodies.
More than 400 people have been evacuated from the area. Residents of housing blocks under one pillar were ready to move back, but were then told it was cracking and that their homes were at risk.
There are 15 people being treated in hospital, nine of them in a serious condition, according to the local prefecture.