A nuclear power plant affected by a massive earthquake is facing a possible meltdown, an official with Japan's nuclear safety commission said Saturday.

Ryohei Shiomi said that officials were checking whether a meltdown had taken place at the Fukushima Daiichi power plant's Unit 1, which had lost cooling ability in the aftermath of Friday's powerful earthquake.

Shiomi said that even if there was a meltdown, it wouldn't affect humans beyond a 10-kilometre radius.

Most of the 51,000 residents living within that radius have been evacuated, he said.

Earlier Saturday, Japan declared states of emergency for five nuclear reactors at two power plants after the units lost cooling ability.

Operators at the Fukushima Daiichi plant's Unit 1 scrambled to tamp down heat and pressure inside the reactor after the 8.9-magnitude quake and the tsunami that followed cut off electricity to the site and disabled emergency generators, knocking out the main cooling system.

Some 3,000 people within three kilometres of the plant were urged to leave their homes, but the evacuation zone was more than tripled to 6 miles (10 kilometers) after authorities detected eight times the normal radiation levels outside the facility and 1,000 times normal inside Unit 1's control room.

The government declared a state of emergency at the Daiichi unit -- the first at a nuclear plant in Japan's history. But hours later, the Tokyo Electric Power Co., which operates the six-reactor Daiichi site in northeastern Japan, announced that it had lost cooling ability at a second reactor there and three units at its nearby Fukushima Daini site.

The government quickly declared states of emergency for those units, too.

Japan's nuclear safety agency said the situation was most dire at Fukushima Daiichi's Unit 1, where pressure had risen to twice what is consider the normal level. The International Atomic Energy Agency said in a statement that diesel generators that normally would have kept cooling systems running at Fukushima Daiichi had been disabled by tsunami flooding.

Builders plan for quakes

Kaoru Ishikawa, the Japanese ambassador to Canada, said all the nuclear power plants in Japan were built "bearing in mind the possibility of big earthquakes."

He said all the plants in the affected area were shut down automatically after the quake, but he noted the coolant problem at the Fukushima plant continued.

The International Atomic Energy Agency said Japanese authorities also reported a fire at the Onagawa nuclear plant. The fire has been extinguished.

"They say Onagawa, Fukushima-Daini and Tokai nuclear power plants were also shut down automatically, and no radiation release has been detected," the IAEA said in a statement posted online.

Of Japan's 54 commercial reactors, 10 of them are shut down following the earthquake. Japan gets about 30 per cent of its electricity from nuclear power, and Tokyo Electric has said it will have to cut back on power generation.

With files from The Associated Press