Local media reported that at least three bodies had been found in the Sasago Tunnel, the CBC reported. Officials sdid not know how many people remain trapped inside.
At least seven people went missing when the nearly 5 km (3-mile-long) tunnel collapsed. News agencies, which cited survivors, reported large chunks of concrete falling on top of cars and a vehicle bursting into flames.
The Sasago Tunnel, one of the longest in Japan, began to collapse at 08:00 local time (23:00 GMT Saturday), about 80 km (50 miles) west of Tokyo on a road that links it to the city of Nagoya, the BBC reported.
"A number of charred bodies were confirmed inside. The number of dead is not known,” a spokesman for Yamanashi Prefectural Police told AFP.
Rescue activity to reach those trapped inside was suspended for several hours after engineers warned of another cave-in.
Emergency crews who made it to a third of the way into the tunnel, equipped with breathing apparatus, found up to 70 m (yards) of collapsed concrete panels that had crushed at least two vehicles.
"According to information from local fire authorities, seven people are missing but the number has not been confirmed," an official at the government's Fire and Disaster Management Agency told AFP.
At least, three people were taken to hospital, including one 28-year-old woman who emerged from the tunnel by herself.
A reporter for the Japan’s NHK broadcaster said he was driving through the tunnel as it began to collapse. His car sustained severe damage.
Another survivor told NHK that he saw "a concrete part of the ceiling fall off all of a sudden when I was driving inside. I saw a fire coming from a crushed car.”