At least seven people are missing and several dead after about 150 concrete panels fell from the roof of a tunnel on the main motorway linking Tokyo with central Japan.
Efforts to rescue any survivors trapped inside the tunnel were hindered by heavy smoke after one vehicle caught fire inside the Sasago Tunnel, about 80km outside Tokyo.
Rescuers also temporarily suspended work because of fears of a further collapse.
They were attempting to reach several vehicles believed buried in the rubble, including a truck whose driver was trapped inside and had called his company for help.
The Fire and Disaster Management Agency said five people were confirmed to have been in a car that burned inside the tunnel, and at least one other was in a truck.
However, officials said they could not confirm the exact number of people believed dead.
Executives for Central Japan Expressway Co said the company was investigating why the concrete panels had given way.
A check of the 4.7-km tunnel's roof in September and October found nothing amiss, they said.
The tunnel, which opened in 1977, is one of many in mountainous Japan. The location of the collapse, about 1.7km inside the tunnel, was complicating rescue efforts, reports said.
Police vehicles, fire trucks and ambulances were massed outside the tunnel's entrance.