Radiation at a tank holding contaminated water at a crippled Japanese nuclear power plant has increased by a factor of 18, the plant's supervisor announced Sunday.

The storage tank at the Fukushima nuclear power plant contained a radiation of 1,800 millisieverts per hour near its bottom Saturday, according to the Tokyo Electric Power Company, or TEPCO, as it is commonly called. That amount of radiation would kill an exposed person in four hours. A spokesman for the company also said that a new leak had been detected from a pipe connecting two other tanks.

By contrast, a reading of the same tank taken on August 22 found radiation of just 100 millisieverts per hour.

The Fukushima plant suffered triple meltdowns after the massive earthquake and tsunami in March 2011. TEPCO must constantly cool the reactors with water, and is struggling to contain the waste.

TEPCO recently acknowledged the chronic leaking of radiation-tainted underground water into the Pacific, plus a 300-ton (300,000-liter, 80,000-gallon) seepage from one of more than 1,000 storage tanks. The leak was the firth and worst from a tank since the crisis began.

The tank leak prompted the nuclear authority to upgrade its rating Wednesday to a level-3 "serious incident," from a level 1 on the International Atomic Energy Agency radiological event scale.

"We cannot fully stop contaminated water leaks right away. That's the reality. The water is still leaking in to the sea, and we should better assess its environmental impact," Japan's Nuclear Regulation Authority Chairman Shunichi Tanaka said in a speech in Tokyo. The agency said Thursday that TEPCO would have to monitor the situation closely.

Tanaka said his agency recently set up a team to collect data more systematically and comprehensively to assess the extent of contamination and evaluate the impact on the ocean.

Scientists have said contamination tends to be carried by a southward current and gets largely diluted as it spreads into the sea.

The Associated Press and Reuters contributed to this report.