At least two people have died and around 20 others are still missing, local media reported on Saturday from Shizuoka prefecture, south-west of Tokyo.

Two women found in "cardiac and respiratory arrest" were later pronounced dead. The whereabouts of about 20 people were unknown after the incident in the coastal town of Atami, local media reported.

The masses of mud flattened everything in their path: electricity pylons, cars, entire residential buildings that collapsed like a house of cards, and streets that sank into the mud.

"The earth slid to the front of the shop. It sounded like an excavator smashing a house," an employee of a glass studio in Atami told reporters. Cars parked in front were washed away, he said.

The mudslide was estimated to be travelling at about 40 kilometres per hour, Professor Motoyuki Ushiyama of the Shizuoka University Centre for Integrated Research and Education of Natural Hazards told Kyodo news agency.

Japanese television broadcast footage of citizens capturing the moment on Twitter when the black wave of mud suddenly burst from a hillside through several houses taking everything with it.

The mudslide covered a wide area and slid close to the coast. The two victims with no signs of life were found near a harbour.

Rescue workers were searching for the missing with the support of soldiers who had been called in. Due to adverse weather conditions, helicopters could not be used in the search and rescue operation at the moment, said Prime Minister Yoshihide Suga. He had previously convened a crisis team in the capital.

At least 10 homes were destroyed. According to the mayor of Atami, Sakae Saito, up to 300 buildings were damaged.

Some 25,000 people were told to move to safe accommodation as the government declared a state of emergency in the area.

Power supply was interrupted in thousands of households.

People in Atami have already begun the clean-up work.

The mudslide occurred after heavy rainfall along Japan's Pacific coast. The meteorological agency in Tokyo also warned of heavy rains in large areas of the country for the next few days.

In the meantime, residents in other places along the country's Pacific coast were warned to take shelter from rising rivers, flooding and possible landslides.

The downpours already led to an interruption in rail services, including the high-speed bullet train.

In the past 10 years, according to official data, there have been an average of almost 1,500 landslides per year in the mountainous island country - almost twice as many as in the previous decade.

Experts ascribe this to increased rainfall due to climate change.

Australian Associated Press