The disclosure by Tepco raises the stakes in a race to complete by next month a system to decontaminate a massive pool of radioactive water at the site that critics see as a growing risk to both the nearby Pacific and groundwater.
A magnitude 9.0 earthquake and the massive tsunami that followed killed about 24,000 people and knocked out the Fukushima plant on March 11, triggering the world's worst nuclear accident since Chernobyl.
The crisis, which has displaced some 80,000 residents from around the plant, prompted a review of Japan's energy policy and growing calls for efforts to step up health monitoring for a crisis now in its 11th week.
Experts from the International Atomic Energy Agency began an inspection on Thursday of equipment damaged by the tsunami at a second nuclear plant, the Tokai complex about 120 km (75 miles) north of Tokyo, as part of an investigation prompted by the Fukushima accident.
A poll by the Asahi newspaper published on Thursday showed that 42 percent of Japanese people opposed nuclear power, up from 18 percent before the disaster.
The survey underscored the public's deepening concerns about nuclear safety and criticism of the way the government and Tepco initially responded to the crisis and how they appeared to have been repeatedly slow in admitting the gravity of the situation.
Although many outside experts had concluded that uranium fuel in three Fukushima reactors had melted down within days of the crisis, Tepco only announced that conclusion this week.
"We have to take seriously the criticism that we haven't done enough to provide and circulate information," Chief Cabinet Secretary Yukio Edano said at a news conference. "But we have never covered up information that we had."
The effort to regain control of the plant relies on pumping massive quantities of water to cool the three reactors that suffered meltdowns and storing the contaminated water in an improvised storage facility. Tepco officials said, however, that the water level in the storage facility had dropped, suggesting a leak.
Environmental groups have focussed on the threat to sea and ground water from the accident. Greenpeace said earlier this month it had collected samples of fish, seaweed and shellfish along the Fukushima coast that showed radiation levels above Japanese safety limits.
Residents of the town of Futaba, forced to evacuate along with others inside a 20-kilometre (12-mile) zone around the plant, were allowed to return briefly to their homes on Wednesday.
A day earlier, residents of the nearby town of Minami Soma had been allowed back to their homes for a two-hour visit wearing hooded white protective suits, masks and goggles.
Video shot by a couple returning home and broadcast on Japanese television showed a ghost town with weeds overrunning a garden and a stray dog barking in the distance.
"It didn't even feel like my own home," one woman told Nippon Television. "I thought I was prepared for that, but I wasn't."