Experts are examining whether the plant lies on a fault line
A radioactive leak at a major nuclear plant in Japan damaged by an earthquake on Monday was worse than previously thought, the plant's operators say.
Owner Tokyo Electric Power company said 50% more radiation was discharged into the sea, following the magnitude 6.8 quake, than was earlier reported.
But the firm insisted the leak was still well below danger levels.
The mayor of nearby Kashiwazaki City has ordered the plant to remain closed indefinitely.
Hiroshi Aida said the plant could not reopen until its safety had been verified.
Meanwhile, Mohamed ElBaradei, head of the UN's atomic agency, has called on Japan to investigate the incident to "make sure that we learn the necessary lesson from the earthquake".
In a statement, the Tokyo Electric Power company (Tepco) said there had been a mistake calculating the radioactive level of water that leaked into the sea.
It was 50% more radioactive than had been announced, the company said.
"But the corrected radioactivity is also below the legal limits and does not affect the environment," Tepco said.
Despite Tepco's reassurances, the incident has triggered public concern and criticism of the company.
The seven-reactor plant suffered more than 50 malfunctions as a result of Monday's earthquake.
As well as the leak, a small amount of radioactive gas was emitted into the atmosphere.
There was also a fire at an electrical transformer, and a number of drums containing low level nuclear waste came open after falling over.
Tepco President Tsunehisa Katsumata has apologised for the incidents.
"I think we can say the size of the earthquake was beyond our expectations," he said as he visited the plant.
"We regret what happened and will strive to make this a power plant that is safe," he said.
The plant is located close to the epicentre of Monday's earthquake, which killed nine people, injured hundreds and flattened scores of homes.
Officials at Japan's Meteorological Agency said that they were examining whether a fault line could stretch underneath the plant.
"We cannot deny the possibility" the plant sat on a fault, the French news agency AFP quoted the agency's Osamu Kamigaichi as saying.