Efforts to rescue survivors trapped inside the tunnel were hindered by heavy smoke after a vehicle caught fire inside the Sasago Tunnel, 50 miles outside Tokyo. Rescuers also temporarily suspended work because of fears of a further collapse.
They were trying to reach several vehicles buried in the rubble, including a truck whose trapped driver 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 after she escaped from the tunnel.
The Fire and Disaster Management Agency issued a statement late last night saying five people were confirmed to have been in a car that burned inside the tunnel, and at least one other was in a truck.
They could not confirm the exact number of people believed dead.
Executives for Central Japan Expressway said the company was investigating why the concrete panels had given way. They said recent checks had found nothing amiss
The tunnel, which opened in 1977, is one of many in mountainous Japan. The location of the collapse, about a mile inside the tunnel, complicated rescue efforts.