Workers at Japan's crippled Fukushima nuclear plant say they have found radiation hotspots but no new leaks after checking hundreds of water tanks.
Some 300 tonnes of toxic liquid is believed to have escaped from one of the tanks, which are used to cool the broken reactors.
Tokyo Electric Power Co (TEPCO), which operates the plant, warned some of it may have flowed into the Pacific Ocean.
In a statement, the company said: "We have confirmed two spots where radiation doses are high."
However, the levels of water in the plant's 300 tanks have not changed and the ground around them was dry, it said.
Tsuyoshi Numajiri, a spokesman for TEPCO, previously said traces of radioactivity were detected in a drainage stream, adding: "We cannot rule out the possibility that part of the contaminated water flowed into the sea".
On Wednesday, nuclear regulators said the leak represented a level-three "serious incident" on the UN's seven-point International Nuclear Event Scale (INES), which measures radiation accidents.
It is the most serious single event since the plant was declared to be in a "state of cold shutdown" - effectively indicating it was under control at the end of 2011.
The meltdowns at the plant in March 2011, triggered by the earthquake and tsunami, were a level seven on the INES scale.
The Chernobyl disaster in 1986 is the only other incident to have been given the most serious ranking.
TEPCO said puddles of water near the leaking tank were so toxic that anyone exposed to them would receive the same amount of radiation in an hour that a nuclear plant worker in Japan is allowed to receive in five years.
The utility did not have a water-level gauge on the 1,000-tonne container, which experts say would have made it more difficult to detect the problem.
Sky's Lisa Holland, in Fukushima province, said: "People here say they're not about to leave town and the older generation say that, frankly, it's too late. They say that if there are health risks, they'll put up with them.
"Among the younger generation, perhaps those with children, lots of them have families who have lived here for many years. They have jobs and homes here and they simply don't want to relocate to other parts of the country.
"Many people in Japan don't want the kind of testing that people in other countries might call for because they don't want the answers that might bring."
The safety checks at Fukushima come after Nuclear Regulation Authority (NRA) chairman Shunichi Tanaka voiced concern about the safety of containers at the site.
He said: "We must carefully deal with the problem on the assumption that if one tank springs a leak the same thing can happen at other tanks."
TEPCO, which faces huge clean-up and compensation costs, has struggled to cope with the disaster.
In July, the company admitted for the first time that radioactive groundwater had been leaking outside the plant.
This month it started pumping it out to reduce leakage into the Pacific.