Workers at Japan’s stricken nuclear plant on Thursday pumped nitrogen into a crippled reactor in a bid to prevent a possible explosion, as the government mulled widening an exclusion zone around the site.

With the crisis at the Fukushima Daiichi plant approaching the end of its fourth week, operator Tokyo electric power said it was concerned a build-up of hydrogen gas at the No. 1 reactor could cause another explosion at the site.

And a week after the UN's nuclear watchdog said the 20-kilometre (12-mile) evacuation zone around the stricken plant should be increased, chief government spokesman Yukio Edano indicated Tokyo was considering widening the zone.

"The existing safety standards for local residents are that an evacuation order is issued if there is a possibility that they might receive radiation 50 millisieverts or above," he said.

"The standard assumed that a high level of radiation is emitted temporarily."

"We are discussing how best to issue evacuation orders based on data and standards for accumulative radiation," Edano said.

He added a procedure to inject nitrogen gas into the housing around reactor No. 1 was "proceeding smoothly".

Tepco said they were concerned that massed hydrogen inside the vessel could mix with incoming oxygen, creating an explosion.

Experts say the risk of a detonation could rise as the nuclear fuel rods cool and as the steam inside the containment vessel condenses into water, reducing pressure inside the unit and drawing air in through cracks.

Workers began pumping in nitrogen, an inert gas abundant in the atmosphere, which they hope will displace the oxygen. The process to inject 6,000 cubic metres (210,000 cubic feet) will take around six days, Tepco said.

"Workers started injecting nitrogen gas at 1.31 am (1631 GMT on Wednesday). Since the pressure level went up, they confirmed that the gas was successfully going into the container," said a spokesman with Japan's nuclear safety agency.

Tepco said it was also planning to inject nitrogen gas into reactors number 2 and 3 as a preventative measure.

In the days after the earthquake and tsunami crippled the plant, large explosions resulted from hydrogen accumulation near the reactors, damaging the outer buildings housing them.

Around 3,400 people are unaccounted for along the 40-kilometre stretch of coast covered by the exclusion zone and on Thursday, around 300 police began searching for bodies in the the outer 10-kilometre band of the zone.

Television pictures showed officers in full body suits entering the area, while a police spokesman said all officers were armed with radiation meters.

The plant has emitted radioactive material into the air, contaminating drinking water and farm produce, with radioactive iodine above legal limits detected in vegetables, dairy products and mushrooms.

Nuclear concerns continue to distract from the March 11 disaster that has left more than 12,500 dead and over 15,000 missing. Around 160,000 people remain in evacuation centres.

A huge relief operation continues, with tens of thousands of Japanese troops assisted by around 15,000 US personnel.

Reports said US secretary of state Hillary Clinton was to visit Japan, a long-time security ally, although relations have been strained since the centre-left Democratic Party of Japan came to power in September 2009.

Support from allies will be welcomed in Japan, where concern was rising over the possible contamination of seafood following a spike in levels of radioactive iodine-131 in seawater outside the plant.

"If the situation worsens we don't know what the outlook will be," a manager at a popular sushi chain told AFP, saying the number of customers was down by about a third compared with normal times. "It's very scary to think about."

The Bank of Japan on Thursday warned of pressures as a result of the triple disaster and bolstered funding for quake-hit areas, unveiling a 1.0 trillion yen ($11.7 billion) scheme to keep banks in affected areas liquid.

The BoJ also downgraded its view of the economy due to last month's disasters.

The government estimates the total cost from collapsed or damaged houses, factories and infrastructure such as roads and bridges at 16-25 trillion yen over the next three years.