The Japanese nuclear safety agency said on Wednesday it is clueless why radiation in waters off Fukushima nuclear plant has gone more than 3,000 times above the legal limit.

The figures are rising further. We need to find out as quickly as possible the causes and to stop them from rising any higher, an official from the nuclear safety agency said on Wednesday.

However, the official downplayed the impact of high level radiation in sea water, saying the impact on human beings will be reduced considerably as Iodine 131 has a half-life of only eight days.

Iodine 131 has a half-life of eight days, and even considering its concentration in marine life, it will have deteriorated considerably by the time it reaches people, the official said.

The Tokyo Electric Power (TEPCO), which runs the Fukushima plant, announced on Wednesday it will decommission four reactors at the plant. Two other plants had been scrapped already.

Japanese government had evacuated people residing in a 20-kilometer radius of the plant, but authorities said on Tuesday they could consider extending the evacuation zone further out. Prime Minister Naoto Kan said the country was in maximum alert over the nuclear crisis.

The International Atomic Energy Agency (IAEA) has said the situation in Japan remains very serious.