A major international mission to investigate Japan's wrecked nuclear complex has begun as new information emerged suggesting that two more of its reactors suffered a meltdown.
That would mean that all three of the most troubled reactors at the plant have suffered partial meltdowns.
The team of UN nuclear experts met Japanese officials and planned to visit the Fukushima Dai-ichi plant in coming days to investigate the worst nuclear accident since Chernobyl in 1986 and assess efforts to stabilise the complex by Tokyo's self-declared deadline of early next year. The team from the Vienna-based International Atomic Energy Agency will report to an international conference in Vienna on June 20.
Meanwhile, the plant operator, Tokyo Electric Power Company, released a new analysis suggesting that fuel rods in the plant's Units 2 and 3 mostly melted during the early days of the crisis, which had been suspected but not confirmed.
In addition, some chunks of the fuel appeared to have entered the outer containment chambers, causing some damage.
That suggests that the severity of the accident was greater than officials have acknowledged. Tepco announced similar findings last week about Unit 1.
The new revelations indicate that earlier official assessments may have been too optimistic, said Goshi Hosono, director of Japan's nuclear crisis task force.
Fuel in three of the plant's six reactors started melting just hours after the March 11 tsunami knocked out cooling systems, prompting huge releases of radiation into the atmosphere - about one-tenth of the radiation released from the Chernobyl disaster in 1986, according to a government estimate.
The melted fuel rods, which appear to have fallen into a lump at the bottom of each of the three pressure vessels, currently pose no immediate problem because they are mostly covered with water being pumped into the chamber and are at temperatures far below dangerous levels, officials say.
The plant is still leaking radiation, but at much lower levels than immediately after the accident, and Japanese officials hope to bring the entire plant to a "cold shutdown" - halting all radioactive leaks - by January at the latest.