The cavity has expanded to 40 metres (130 feet) across, almost the length of an Olympic swimming pool, since opening up in a city just north of Tokyo on Tuesday morning, officials said.

The growing hole could be the result of corroded sewage pipes, according to authorities in Yashio.

"It is an extremely dangerous condition," local fire chief Tetsuji Sato told reporters on Thursday at the traffic intersection where dozens of rescuers have been working around the clock.

"We are planning to construct a slope (to access the hole) from a safer spot so that we will be able to send heavy equipment," he said.

He added that groundwater was leaking inside and that the hole was "continuing to cave in".

No communication has been had with the driver since around midday Tuesday, with soil and other debris now covering the cabin of his lorry in Yashio.

ERODING WALLS 

The punctured pipes "potentially allowed the surrounding soil to flow in and the space under the ground to hollow out", Daisuke Tsutsui, a Saitama prefectural official, told AFP on Thursday.

Authorities hoped to complete the 30-metre slope on Friday, but a local official said it may take several days.

The operation has been aggravated by the inner walls of the hole -- now around 10 metres (30 feet) deep -- continuing to erode, preventing rescue workers from staying inside it for long.

Initially, the hole was around five metres in diameter but it has since combined with a much larger cavity that opened during the rescue operation on Tuesday night.

As the sinkhole has expanded, heavy chunks of asphalt have occasionally fallen in, preventing rescue workers from going near the chasm.

This has also made it dangerous to place heavy machinery nearby.

The 1.2 million people living in the area have been asked to cut back on showers and laundry to prevent leaking sewage from making the operation even more difficult.

"Using toilets is difficult to refrain from, but we are asking to use less water as much as possible," an official told AFP.

Some sewage water in the area was collected and released to a nearby river to reduce the runoff into the hole.

"It feels rather abnormal that the search is taking this long. I wonder if he could've been saved much sooner," Takuya Koroku, a local factory worker, told AFP on Thursday.

"I'm scared to go nearby," the 51-year-old added.

© Agence France-Presse