At least seven people were feared trapped and an unknown number of cars were ablaze after a 100-metre long section of highway tunnel collapsed in Japan on Sunday, media and emergency services said.

At least three vehicles were crushed and three people were injured in the accident, public broadcaster NHK said.

The accident occurred on Tokyo-bound lanes inside the Sasago tunnel on the Chuo Expressway, some 80 kilometres (50 miles) west of the capital, at around 8:00 am (2300 GMT Saturday), an official at the expressway traffic police said.

"We don't know exactly how many vehicles are on fire," he told AFP by telephone.

NHK footage from inside the tunnel showed a white ambulance and several firefighters wearing protective gear, working in an section shrouded in smoke. A number of cars with their lights flashing were also seen.

Several Japanese media reported that a 100-metre (yard) section of tunnel had caved in, but there was no immediate confirmation from authorities.

Kyodo News reported that firefighters had said there were "at least five" people trapped in the tunnel.

A 28-year-old woman was taken to hospital by ambulance after she emerged from the 4.3-kilometre-(2.7-mile)-long tunnel by herself, the traffic police official said.

The 28-year-old women told the fire rescue unit she had been riding in a rented van with five other people, fire department official Kazuya Tezuka told AFP by telephone.

"I have no idea about what happened to the five others. I don't know how many vehicles were ahead and behind ours," she was quoted as saying.

The tunnel, which passes through hills not far from Mount Fuji, is one of the longest in Japan and sits on a major highway connecting Tokyo with the centre and west of the country.

NHK said two other women were rescued from inside the tunnel and that the extent of their injuries was unknown.

A reporter for NHK said he happened to be driving through the tunnel on his way to Tokyo when it started to disintegrate.

"I managed to drive through the tunnel but vehicles nearby appeared to have been trapped," he said. "Black smoke was coming and there seemed to be a fire inside the tunnel."

The thick smoke inside the tunnel was hampering rescuers' attempts to reach the caved-in point, some two kilometres (1.3 miles) from the Tokyo-side exit, the fire department's Tezuka said.

"The tunnel's smoke ventilation system is malfunctioning and we can't see anything one metre in front," he told AFP.

Aerial footage on NHK showed several red trucks from the local fire department waiting outside the Tokyo side of the tunnel.

Dozens of people were seen waiting at an expressway bus stop just outside the exit, and were believed to have exited from the tunnel, NHK said.

A man in his 30s, who was just 50 metres ahead of the caved-in spot when the accident happened, recounted details of the terrifying experience.

"A concrete part of the ceiling fell off all of a sudden when I was driving inside. I saw a fire coming from a crushed car. I was so frightened I got out of my car right away and walked one hour to get outside," he told NHK.

"The traffic was not so heavy," he added.

A stream of people was seen coming out of the other exit after abandoning their vehicles in the tunnel, the broadcaster said.

Japan has an extensive and well-maintained network of thousands of highways throughout its mountainous terrain, usually several hundred metres long. Millions of cars use the network every day.