"Very weak steam" has been detected from three small vents at about the middle level of the mountain's northeastern side in the past five days, the Meteorological Agency said.

"We have not detected any type of volcanic gas such as hydrogen sulphide," the agency said in a statement.

The announcement came hours after a massive earthquake measuring 8.0 on the Richter scale rocked the northern Japan island of Hokkaido, reportedly injuring more than 400 people. The two events were deemed unrelated.

Two of the holes were found in a small area of subsidence, 15 metersfeet) by 10 meters (33 feet) and up to 30 centimeters (12 inches) deep. There were also three smaller patches of subsidence in the area, the statement said.

"We do not assume that the subsidence of ground surface and the steam activity are directly related to any volcanic activity," the agency said, adding it would continue to monitor temperatures around the vents.

The area around Mount Fuji is a hotbed of geo-thermal activity supporting a thriving "onsen" hot-spring resort industry.

The 3,776 meter (12,461 feet) high Mount Fuji, last erupted in 1707 and has been under scrutiny since 2000 when low-frequency tremors started to be felt there, confirming that the mountain is still an active volcano.

Last year, the government estimated that the potential financial cost from a future eruption of Mount Fuji, some 100 kilometers (60 miles) west of Tokyo, could reach 20 billion dollars.

According to the government estimate, if the near perfect conical volcano erupted, it would paralyse traffic, including the bullet train system, and affect more than 28.3 million people.

TERRA.WIRE