A powerful earthquake tore up hills, fields and roads in northern Japan on Saturday, killing at least six people, injuring around 100 more and trapping guests at a levelled resort hotel.

The earthquake, which measured 7.2 on the Richter scale, also caused a small leak of radioactive water from a power plant, although the company said there was no cause for public concern.

Japan deployed nearly 800 troops to the largely agricultural region in Miyagi and Iwate prefectures, where military helicopters plucked to safety residents, many of them elderly, who were suddenly cut off from the world.

Landslides snapped highways, which abruptly turned into cliffs of falling mud and dirt, and clogged rivers to create a series of "quake lakes."

"I was driving my car when the earthquake hit," said Makoto Katsurashima, 72. "I just turned white as I saw the road disappear before my eyes a few metres (yards) away."

The quake, which struck just eight kilometres (five miles) underground, was strong enough to shake buildings in Tokyo, 350 kilometres (220 miles) to the south, and was followed by around 160 aftershocks.

Dozens of residents flocked in the evening to makeshift shelters set up in public buildings, either out of fright or because power and running water were cut off to their homes.

"The top priority is to save lives," Prime Minister Yasuo Fukuda said in Tokyo as he dispatched the military and his disaster minister. "We're doing our best in rescue operations."

Six people were killed and another 90 were injured, officials said, while public broadcaster NHK put the number of injured at 162.

The dead included two construction workers, aged 53 and 54, caught in a landslide, with their bodies retrieved hours afterwards.

"One of them for a while hung onto a tree, but then he fell with the tree," said an official at the construction project.

The other dead included a 60-year-old man who rushed out of his home in panic and was hit by a truck.

Twelve people remained missing including three foreigners, whose nationalities were unclear, who were out camping.

The earthquake tore to pieces a hot-spring resort, which turned into a pile of wooden rubble with access cut off by a landslide.

Five people were rescued from the Komanoyu hotel in a remote scenic forest, two of them with broken bones, but several remained missing, police said.

Ayako Inomata, whose daughter worked there, said she took a helicopter to the resort hotel and found that 31 customers and workers were safe.

"I was so relieved because today I couldn't get through on her mobile or on her landline. But my daughter and her colleagues and other customers there looked OK," Inomata said.

Kyoichi Suzuki, a 50-year-old beekeeper, said he was just 100 metres away from a landslide that buried a car.

"I escaped by a hair's breadth," he said with relief afterwards. "If I had been in that car, I would have been killed."

Japan endures about 20 percent of the world's powerful earthquakes and has built an infrastructure intended to withstand the impact of tremors.

Tokyo Electric Power Co. said that 14.8 litres (3.8 gallons) of water came out of a pool in which radioactive equipment is stored at a reactor in Fukushima prefecture, but the company said there were no risks to the public.

Japan's land ministry said around five "quake lakes" were formed when landslides blocked rivers. But it said it did not expect dangers from the lakes, which posed a major risk after last month's devastating earthquake in China's Sichuan province.

Masanori Oikawa, a local official in Oshu, said that people in his town were responding calmly, even though they were in shock.

"The jolt was so strong that I couldn't stand without holding onto the wall," he said. "We saw electric poles swinging and the walls of homes were damaged."

"We're used to earthquakes, but this was really scary."