A woman living about a kilometer (half a mile) from the blast site said explosion caused panic because no one knew what it was.

"I was in the house and I heard a huge bang and screams from the window. I saw a huge plume of smoke," Crisina Felicani told Sky TG24. "There was panic in the neighborhood, because it wasn't clear what happened."Many people had evacuated the area, due to the thick black smoke, but also while firefighters checked to see if buildings where windows had been blown out were safe to enter.

"It almost would have been an ordinary accident except for the highly flammable material this truck was transporting," firefighter Carlo Cardinali told Sky.

Authorities said they were working to finish the on-sight investigation so highway officials could reclaim the damaged road and adjacent lanes and begin to rebuild the roadway, which has long been subject to major traffic backups.

The accident closed down a key section of a major north-south highway that is heavily used year-round, but especially as Italy heads into next week's peak summer holiday travel period.

Italy's infrastructure and transport minister, Danilo Toninelli, said authorities were working to ensure that traffic would be able to move smoothly "on such a critical stretch."

In a separate vehicle accident in the southern Italian region of Puglia, 12 farm workers were killed Monday when the van they were traveling in collided with a truck and overturned, ANSA reported.

The accident was similar to one on Saturday, when a truck laden with tomatoes crashed into a white van and four migrants died.

AP