The Japanese government says it does not know when its nuclear crisis will end as radiation levels surge near the stricken Fukushima power plant.

The amount of radioactive iodine in the sea near the plant has increased tenfold since Tuesday, reaching more than 1,000 times the legal limit.

Inside the plant, workers have found pools of water with radiation levels 10,000 times higher than normal.

"It is becoming very important to get rid of the puddles quickly," said Hidehiko Nishiyama, an official at the nuclear safety agency.

One of the worst-case scenarios at reactor No. 3 would be that the fuel inside the reactor core - a volatile uranium-plutonium mix - has already started to burn its way through its steel pressure vessel.

"Highly radioactive water is flowing inside the buildings and then into the sea, which is worrying for fish and marine vegetation," said Olivier Isnard, an expert at France's Institute for Radiological Protection and Nuclear Safety.

"One hypothesis is that the reactor vessel is breached and highly radioactive corium is coming out."

Fire engines have hosed thousands of tons of seawater onto the plant in a bid to keep the fuel rods inside reactor cores and pools from being exposed to the air, where they could reach critical stage and go into full meltdown.

Urgent efforts were underway to drain the pools after several workers suffered radiation burns while installing cables as part of efforts to restore the critical cooling systems.

The new safety worries further complicated efforts to bring the ageing facility under control and raised fears that the fuel rod vessels or their valves and pipes are leaking.

Several hundred metres offshore in the Pacific Ocean, levels of iodine-131 about 1,250 times the legal limit were detected on Saturday, plant operator Tokyo Electric Power Co (TEPCO) said.

Mr Nishiyama said drinking a 500ml litre bottle of fresh water with the same concentration would expose a person to their annual safe dose.

Officials insist there is minimal risk to humans and marine life.

"Generally speaking, radioactive material released into the sea will spread due to tides, so you need much more for seaweed and sea life to absorb it," Mr Nishiyama said.

Because iodine-131 decays relatively quickly, with a half-life of eight days, "by the time people eat the sea products, its amount is likely to have diminished significantly," he said.

However, TEPCO also reported levels of caesium-137 - which has a half life of about 30 years - almost 80 times the legal maximum. Scientists say both radioactive substances can cause cancer if absorbed by humans.

- ABC/AFP