Rescue officials have started laying a slope into an expanding sinkhole north of Tokyo as they prepare to launch a full-fledged search for a truck driver.

The 74-year-old driver remains unaccounted for after his truck plunged into a hole that emerged in a road in Yashio City, Saitama Prefecture on Tuesday.

The cave-in later connected with another collapse nearby and expanded into a depression measuring about 20 meters in diameter.

Prefectural officials believe the first cave-in was caused by the rupture of a sewage pipe roughly 10 meters underground.

Collapses have intermittently continued inside the merged depression, where soil is mixed with structural debris and other rubble.

Firefighters started making the slope on Thursday to enable heavy duty machinery to clear away soil and rubble. They expect to complete the slope, which will measure around 4 meters by 30 meters, within several days.

Prefectural officials are also working to prevent rain and other water from flowing into the hole.

The firefighters say these efforts will enable them to conduct a full operation to look for the driver.

The head of a local fire station, Sato Tetsuji, said conditions have made it difficult to search for the driver as the total collapsed area is now several times the size of the first hole.

He added his team will try to locate the driver as soon as possible while taking thorough measures to ensure safety.