Japan's military has taken action in the search for the missing boy who was left by his parents in a bear-infested forest as punishment for misbehaving, officials said.

In a statement published in The Guardian, 75 soldiers from the Ground Self-Defense Force (SDF) joined the 130-strong rescue group consisting of police, firefighters, rescue personnel and volunteers.

Despite the addition in the search team, there is still no sign of the boy.

"We still have no clue. We just do not know where he went," Satoshi Saito, spokesperson for the SDF rescue team, told CNN.

"Our fatigue level is peaking now. But we will continue to search for the boy," he added.

Seven-year-old Yamato Tanooka went missing in Hokkaido forest in northern Japan on Saturday evening after his parents made him get out of the car and left him in the woods as a punishment for misbehaving.

The boy's parents originally told the police that their son got lost while walking to gather vegetables, but later admitted to deliberately leaving him behind as a punishment for throwing stones at people and cars.

The family drove back five minutes later but Tanooka was gone.

The local town of Nanae requested military support after failed search attempts.

"We have already covered the same areas over and over again," spokesman Mitsuru Wakayama said in a report. He added that even an adult would not be able to travel along such an extensive area by foot.

Thunderstorms are also hampering the search activities last Tuesday, but clearer weather the next day will allow rescuers to search until nightfall, officials said.

According to Saito, officials are considering the possibility that the boy was picked up by someone on the road, or that he could have wandered into the forest by himself.

Tanooka was last seen in a black T-shirt, jeans and red sneakers, and this compounded fears as overnight temperature in the area can fall as low as 7 degrees Celsius. He did not have any food or water or even a mobile phone.

"The military personnel will scour the mountain slope," said Saito. "Unless he started climbing the mountain, he would have hit a main road after walking for two to three kilometers in any other direction."