At least three of the facility's seven underground storage pools were discovered to be seeping thousands of gallons of radioactive water into the soil, The New York Times reported Wednesday.
"The Fukushima Daiichi plant remains in an unstable condition, and there is concern that we cannot prevent another accident," Shunichi Tanaka, chairman of the Nuclear Regulation Authority, said at a news conference, the Times reports.
He said his agency had ordered Tokyo Electric Power Co., operator of the plant, "to work on reducing some of the biggest risks, and we as regulators will step up monitoring."
Economy, Trade and Industry Minister Toshimitsu Motegi also told Tepco President Naomi Hirose in a meeting Monday that "if these kinds of incidents continue to occur, the very process toward decommissioning the (crippled) reactors could be affected."
The minister ordered Tepco to take steps so that radioactive water leaking from the underground storage tanks will not flow into the adjacent Pacific Ocean.
Hirose, visiting Fukushima on Wednesday, admitted his company had not yet determined the cause of the water leaks from three of the underground storage tanks, the Asahi Shimbun reports.
While four of the tanks currently hold 23,600 tons of radioactive water, Tepco said it will move 7,100 tons of the water to existing surface storage tanks.
Hirose dismissed speculation that radioactive water could be released into the Pacific Ocean.
"That will absolutely never happen," he said.
Meanwhile, experts have questioned Tepco's ability to manage the deteriorating situation and the cleanup and decommissioning of the plant's reactors, expected to last decades.
"It's become obvious that Tepco is not at all capable of leading the cleanup," Muneo Morokuzu, a nuclear safety expert at the Tokyo University Graduate School of Public Policy told the Times, noting that the plant originally should have had a more permanent solution that would reduce the flow of contaminated water into the plant.
Tepco "just doesn't have the expertise," Morokuzu said,
"The government needs to step in, take charge and assemble experts and technology from around the world to handle the decommissioning instead."
Only two of Japan's 50 operable nuclear reactors are online, following shutdowns ordered after the Fukushima crisis.
Separately on Wednesday, the Nuclear Regulation Authority released its proposal for new regulation standards for reactors.
Those standards, expected to be enforced beginning in July, set high and costly hurdles and some of the required measures could take years for plant operators to install. As a result, most of the 48 reactors currently offline are not likely to resume operations over the next few years.