Tokyo (CNN) -- An unknown number of cars were trapped early Sunday after a tunnel collapsed some 50 miles (80 kilometers) west of Tokyo, police said.
The cause of the collapse was not immediately clear, nor was the number of people potentially affected.
The Sasago tunnel is located on the Chuo Expressway. According to Otsuki police, the cave-in occurred on the Tokyo-bound lanes.
The road was temporarily closed and authorities were working to rescue victims, police said.
Japanese public broadcaster NHK broadcast images from the tunnel, showing smoke rising and a blue car with its side smashed in. Emergency vehicles were on scene.
A journalist from NHK was driving through the tunnel at the time of the collapse. He told the broadcaster his car was damaged, but he was able to make it out unhurt.