At least two people have been killed and 20 more missing after a landslide triggered by torrential rains tore through a resort town in central Japan on Saturday, destroying more than 80 homes and affecting about 21,000 households.

Japan Meteorological Agency said the disaster occurred about 10:30 am local time in the Izusan district of Atami city, Shizuoka Prefecture, and the mudslide washed down soil about 2 kilometers to the sea.

"Because of the heavy rain, the ground loosened and the mudslide occurred," said Heita Kawakatsu, governor of Shizuoka. "It picked up speed and swept away houses together with people."

Atami, a coastal city known for its hot spa resorts, is about 100 km southwest of Tokyo.

A video clip posted on social media showed mud sliding down a mountainside, destroying several homes in its path as several people ran for their lives. A woman could be heard in the video saying "This is scary" seconds before the flash flood hit.

As search-and-rescue efforts began, local authorities issued the highest level of evacuation alert to about 21,000 households in Atami, and the Ground Self-Defence Force of Japan had dispatched about 30 personnel to the area in response to a request from the Shizuoka government.

Horrible sound

"I heard a horrible sound and saw mud heading down as rescue workers were urging people to evacuate, so I ran to higher ground," said one witness to the public broadcaster NHK.

"The mudslide came really close, right in front of my house," another survivor told NHK. "I could hear it coming before I saw it. I couldn't get away, so I climbed up a ladder."

Prime Minister Yoshihide Suga has set up a task force in his office to collect information and urged officials to work with local authorities on ascertaining the damage and their needs, and calling on the public to be ready to evacuate and "take action to protect lives".

The Chinese embassy in Tokyo issued a mudslide alert on Sunday, calling Chinese citizens to "pay close attention to disaster information" and "take effective protective measures".

"In case of emergencies or difficulties, please seek help from us or relevant Japanese agencies as soon as possible," the embassy said.

Yoshiharu Ishikawa, a landslide expert and an honorary professor at the Tokyo University of Agriculture and Technology, warned people to be cautious.

"Atami city is bordered by mountains with steep slopes, many covered with a layer of volcanic soil, which can flow down a slope easily after absorbing water," Ishikawa said.

The risk of a mudslide does not diminish even after rain subsides because it takes time for the ground to absorb water, he said.