Radiation in water leaked from the world's largest nuclear plant after a major quake in Japan is 1.5 times higher than initially estimated.

Mohamed ElBaradei, Chief of the International Atomic Energy Agency, urged Japan to be transparent in its investigation of the accident, adding that the UN nuclear watchdog is ready to join Japan to investigate the accident involving minor radiation leaks.

Although the reactor structure had not been damaged, it was necessary for Japanese authorities to make a thorough investigation of the incident, ElBaradei emphasized.

As the showdown escalated over a long list of problems at the Kashiwazaki-Kariwa plant, the world's largest in terms of power output capacity, the mayor of nearby Kashiwazaki city ordered the facility shut down until its safety can be confirmed.

Tsunehisa Katsumata, president of plant operator Tokyo Electric Power Co., toured the site Wednesday morning, declaring it a mess.

After Katsumata's plant tour, TEPCO announced that a leak of radioactive water into the Sea of Japan was actually 50 percent bigger than initially announced Monday night. But the levels were still well below danger levels, it said.

The earthquake of magnitude 6.8 caused 50 problems at the plant, including emission of a small amount of radioactive material into the atmosphere.

About 1,200 liters of water also leaked into the sea from a reactor at the plant, and a blaze began in an electricity transformer.