A state of emergency has been declared at two of Japan's nuclear power plants following the country's strongest earthquake on record and officials warn that small levels of radiation are likely to leak from both plants.

Officials have evacuated the areas around both plants in a 10-kilometre radius.

At one plant, radiation inside one reactor has jumped to 1,000 times the normal level, but officials stressed there was no "immediate health hazard."

Both plants suffered critical failures in their backup generators used to power cooling systems. A lack of power to the cooling systems results in a pressure buildup beyond what the nuclear reactors are designed to handle.

The first plant to suffer a power loss was the Fukushima Daiichi facility near the city of Onahama, about 260 kilometres north of Tokyo.

Pressure began to build inside one of the plant's reactors, prompting the government to declare a state of emergency for the first time ever at a nuclear facility. Hours later, another state of emergency was declared at a second nuclear power plant, again over a problem with its cooling system.

Plant operator Tokyo Electric Power Co. said it had lost the ability to cool three of four nuclear reactors at another power plant, Fukushima Daini, near the towns of Tomioka and Naraha.

Five nuclear reactors are affected by the emergency declarations. All have been shut down, but the core of a reactor remains hot for about two days after it's shut down.

Early on Saturday local time, the country's nuclear safety agency said that radiation had seeped from the buildings. Levels were eight times higher than normal outside the plant's front gate.

Between the two troubled nuclear facilities, which are about 15 kilometres apart, more than 40,000 people have been urged to leave the area.

Pressure inside one of six boiling water reactors at Fukushima Daiichi rose to twice the normal level, Japan's nuclear safety agency said. Following the evacuation order, the government said the plant would release slightly radioactive vapor in the hopes of averting a possible meltdown.

"With evacuation in place and the ocean-bound wind, we can ensure the safety," Chief Cabinet Secretary Yukio Edano said at a news conference early Saturday.

But a continuing power outage delayed plans to release the vapor and ease pressure at Fukushima Daiichi's No. 1 reactor.

Ippo Maeyama, a defence ministry official, said dozens of soldiers trained to deal with chemical disasters were deployed to the Fukushima Daiichi plant in the event of a radiation leak. The government also deployed four vehicles intended for use in atomic, biological or chemical warfare.

The International Atomic Energy Agency (IAEA) said it had declared a heightened state of alert for the plant but that no leaked radiation had been detected there following the shut-down.

"The IAEA is seeking further details on the situation at Fukushima Daiichi and other nuclear power plants and research reactors, including information on off-site and on-site electrical power supplies, cooling systems and the condition of the reactor buildings," the IAEA said in a statement.

Officials said there were no reports of injuries or leaks at any of the affected nuclear facilities.

However, Danny Eudy, a 52-year-old American technician at the plant, told his wife that he and other workers were "waiting to be rescued, and they're in bad shape," she said to The Associated Press by phone.

Eudy called his wife after the ordeal and told her the quake shook the plant violently.

"Everything was falling from the ceiling," she said, and then the tsunami hit the area.

Elsewhere, a fire reportedly broke out at a nuclear plant in the hard-hit Miyagi prefecture. Tohoku Electric Power Co. said the fire had been extinguished and the cause is under investigation.

Another reactor in the same area was leaking water.

With files from The Associated Press