That would mean that all three troubled reactors at the plant have had their cores mostly melted down.
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 stabilize the complex by Tokyo's self-declared deadline of early next year.
Meanwhile, the plant operator, Tokyo Electric Power Co, 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 inner containment chambers, or drywell, 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.
"We should have made a more cautious damage estimate based on a worse scenario," he said.
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.
In the meantime, 80,000 people remain evacuated from homes around the plant. Many are living in school gymnasiums. A handful of stalwarts have defied government orders and refused to leave.
"TEPCO caused such a horrible disaster. Leaving my home means I have lost to TEPCO," said Naoto Matsumura, a 51-year-old rice and vegetable farmer who has stayed at his home despite radiation concerns and a lack of electricity and running water.
"Certainly, life is not comfortable at all," he said. "But I will not give up."
Violators of a 20km exclusion zone can face fines of up to 100,000 yen ($US1200) or 30 days of detention, but no officials have moved to arrest him, he said.
The team from the Vienna-based International Atomic Energy Agency conferred on Tuesday with Japan's economy and trade minister, who oversees - and promotes - the nuclear industry, and will visit Japan through June 2 before reporting to an international conference in Vienna on June 20.
Michael Weightman, leader of the IAEA team, said the delegation would "seek information to see how the world can learn lessons from the unfortunate events here."
Hosono said the team had submitted a "long list" of questions, and that authorities would do their best to answer them.
The government also said it was appointing University of Tokyo Professor Yotaro Hatamura, an expert on industrial and other accidents, to head a panel of outside experts to investigate the Fukushima accident.
The crisis has raised serious questions about lax oversight of Japan's nuclear industry and prompted the country to scrap plans to rely on nuclear power for half its electricity needs - up from its current one third.
The quake and tsunami, which left more than 24,000 people dead or missing, also damaged farms, ports and hundreds of suppliers, helping to push Japan's economy back into recession.
Workers at the troubled plant are now trying to figure out what to do with massive amounts of radioactive water leaking from the reactor cores into the reactor and turbine buildings, trenches and pits.
In the latest setback, TEPCO said containers holding radioactive water pumped from those areas were nearly full, raising concerns that the water could overflow and leak into the sea again.
Under a revised roadmap released last week, TEPCO plans to install reprocessing equipment that would pump contaminated water out of the turbine basements where it is pooling and put it back into the reactor as coolant.
The utility also plans to put a special cover over each reactor building to contain radiation releases.
Given the greater-than-expected damage to the reactors, some experts question whether the company will be able to bring the plant under control by early next year.
Of the remaining reactors at the plant, Unit 4 had no fuel rods inside its core at the time of the tsunami, but workers have struggled to keep cool its cache of used fuel rods. Units 5 and 6 were brought into a cold shutdown in late March.
