Tokyo Electric Power Co., the plant's operator, this week confirmed about 79,000 gallons of radioactive water leaked from a storage tank. It warned there may be hundreds more tanks like it on the site of the 2011 meltdown.
Chairman of the Japan Nuclear Regulation Authority Shunichi Tanaka was quoted Thursday by The Wall Street Journal as saying the situation was alarming.
"We cannot waste even a minute," he said. "This is what we have been fearing."
The nuclear watchdog raised the alert level to 3, a serious incident, this week. The meltdown itself was categorized as a level 7 nuclear event, the highest level.
Tepco said Wednesday it was running out of space to store the 105,000 gallons of radioactive water pumped out of contaminated reactors each day.
The utility company said water hasn't yet reached the waters surrounding the facility.
Atsunao Marui, director of research at Japan's National Institute of Advanced Industrial Science and Technology, said the situation is likely to get worse.
"It's important to think of the worst-case scenario," he said.