Radiation at a hobbled nuclear plant in Japan was 10 million times more than normal, officials said.
Workers were evacuated on Sunday from the reactor building in Fukushima to prevent exposure, the plant's operator said.
The high radiation levels were detected at reactor number 2 in water that had accumulated in the turbine housing unit, Tokyo Electric Power Co (TEPCO), the plant's operator, said.
The operator of the facility said radiation in the water of reactor number 2 was measured at more than 1,000 millisieverts an hour, the highest reading so far in a crisis brought on by a massive earthquake and tsunami on March 11.
Officials said the high levels of radiation was probably caused by leakage from reactor vessels.
Japanese engineers have struggled to pump radioactive water from the plant 240km north Tokyo two weeks after it was hit by an earthquake and tsunami.
Engineers trying to stabilise the plant had to pump out radioactive water after it was found in buildings housing three of the six reactors.
Meanwhile, tests by the Japanese nuclear safety agency revealed levels of radioactivity up to 1,850 times the usual level in seawater offshore the crippled plant compared to 1,250 measured on Saturday.
"Ocean currents will disperse radiation particles and so it will be very diluted by the time it gets consumed by fish and seaweed," said Hidehiko Nishiyama, a senior agency official.
Experts say the rise in radioactivity in the water at the reactor does not pose much danger to those outside as long as it is contained safely.
"It depends on where this water's going and what they're doing with it," said Murray Jennex, professor at San Diego State University.
"If it's allowed to run off into the ground and stuff, you're getting a concentration in the ground. If it's going into the ocean, you're getting some accumulation in the ocean."
Speaking to Al Jazeera, George Dracoulis, head of Nuclear Physics department at Australian National University, said: "They have to map the areas, see where the radiation is and sample the sea life and that would determine what they do in the future."
The nuclear crisis has overshadowed a big relief and recovery effort from the magnitude 9.0 quake and the huge tsunami it triggered.
Official death toll from earthquake and tsunami now stands at 10,489, with the number of missing put at 16,621. Nearly a quarter of a million people are living in shelters.
'Failure of communication'
On Thursday, three workers were taken to hospital from reactor number 3 after stepping into water with radiation levels 10,000 times higher than normal. That raised fears about the core's container being damaged.
TEPCO said "the radiation exposure on Thursday occured because there was bad sharing of information".
"We have to apologise. We want to make efforts to share information within the company."
Experts still had to determine where to put some of the contaminated water while engineers were still trying to fully restore the plant's power, the company said.
It said it was now using fresh water instead of seawater to cool down at least some of the reactors after concern arose that salt deposits might hamper the cooling process.
George Dracoulis said that "The issue with using sea water is that it is corrosive, salt in water can become activated and it can cause further contamination."
Two of the plant's reactors are now seen as safe but the other four are volatile, occasionally emitting steam and smoke. However, the nuclear safety agency said on Saturday that temperature and pressure in all reactors had stabilised.
The government has said the situation was nowhere near to being resolved, although it was not deteriorating.
"We are preventing the situation from worsening - we've restored power and pumped in fresh water - and making basic steps towards improvement but there is still no room for complacency," Yukio Edano, the chief cabinet secretary told a news conference.
Radiation levels 40 per cent higher than the yearly limit for the general public have been detected just over 30k from the Fukushima plant.
The government has not told residents outside the 30km radius of the plant to evacuate, or even to stay indoors.
The science ministry says a reading of 1.4 millisieverts was taken on Wednesday morning in Namie Town northwest of the plant.
Someone staying outdoors for 24-hours at that location would exceed the annual limit of one millisievert. The limit is based on a recommendation by the International Commission on Radiological Protection.
The science ministry obtained the reading after monitoring 10 locations outside the 30km zone following reports that relatively high levels of radiation were found outside that area.
Exerts say the amount of radiation detected does not pose a health risk. But they advise residents in the area to stay alert for any possible rise in radiation levels, because the power plant is not likely to stop releasing radiation any time soon.
Yukiya Amano, head of the International Atomic Energy Agency (IAEA), told the New York Times that the emergency "is a very serious accident by all standards" and could go on for weeks.
The IAEA has sent new teams to Japan to monitor radiation and assess contamination of food.
Prolonged efforts to prevent a catastrophic meltdown at the 40-year-old plant have also intensified concern around the world about nuclear power.
Ban Ki-moon, the UN Secretary-General, said it was time to reassess the international atomic safety regime.