Efforts to rescue any survivors trapped inside the tunnel were hindered by heavy smoke after one vehicle caught fire inside the Sasago Tunnel, about 50 miles outside Tokyo.
Rescuers also temporarily suspended work because of fears of a further collapse. They were attempting to reach at least several vehicles believed buried in the rubble, including a truck whose driver was trapped inside and had called his company for help.
"I could hear voices of people calling for help, but the fire was just too strong," said a woman interviewed by public broadcaster NHK after she escaped from the tunnel.
Local media reported that at least three bodies had been found inside the tunnel. However, Norio Furusawa, a spokesman for the Fire and Disaster Management Agency, said he could not confirm that information.
Recent check found no issues
Executives for Central Japan Expressway Co. said the company was investigating why the concrete panels had given way. A check of the tunnel's roof in September and October found nothing amiss, they said.
A woman who escaped from a rental car that was trapped in the 3-mile tunnel told authorities that she was unsure about the condition of five other people who had been in the vehicle with her. Two other vehicles were known to be buried in the rubble, suggesting at least seven people were trapped inside, according to a statement by the Fire and Disaster Management Agency.
It said two people were confirmed injured, one of them moderately.
The tunnel, which opened in 1977, is one of many in mountainous Japan. The location of the collapse, about a mile inside the tunnel, was complicating rescue efforts, reports said.
Police vehicles, fire trucks and ambulances were massed outside the tunnel's entrance.