The plant's operator also said that it was possible that the pressure vessels in the three stricken reactors, which house the uranium fuel rods, had been breached as well. But most of the fuel remained inside the vessels, the company said — far from a more severe nuclear meltdown in which molten fuel penetrates the ground, a calamity known as the “China Syndrome.”

Also Tuesday, a team of experts from the International Atomic Energy Agency, the nuclear oversight body of the United Nations, began an investigation into Japan's handling of the accident, amid criticism that a slow response made matters worse.

“We're here to gather info and to seek to learn lessons that we can apply across the world to improve nuclear safety,” Michael Weightman, the chief nuclear inspector of Britain and the team's leader, said at a meeting with Japan's trade minister.

Tuesday's disclosure by Tokyo Electric Power, the operator of the stricken Fukushima Daiichi nuclear power plant, could delay efforts to bring the plant's reactors under control. Earlier this month, the company released an updated plan to bring all reactors at the plant to a stable state known as a “cold shutdown” in six to nine months. But that goal was based on an understanding that workers could efficiently cool the fuel in the three reactors, a harder task if their inner pressure vessels are breached.

It is also likely to trigger more criticism over what many critics have called a lack of timely disclosure by Tokyo Electric, and by the Japanese government, of important details of the accident. Prime Minister Naoto Kan has apologized in parliament for fanning public mistrust.

“We take this disclosure very seriously. But what's important now is our response,” said Goshi Hosono, who heads the Japanese government's nuclear crisis task force.

“Though we now know the situation is very severe, the fuel still remains inside the reactors,” Mr. Hosono said. “There is no change to our strategy of continuing to cool the reactors until we can bring them to a stable state,” he said.

Experts had long suggested that meltdowns occurred at the three reactors after a 50-foot tsunami knocked out power for their cooling systems, causing the nuclear fuel at the reactors' cores to overheat.

Three other reactors at the Fukushima Daiichi nuclear power plant, about 150 miles north of Tokyo, were not operating at the time of the March 11 earthquake and tsunami.

Last week Tokyo Electric for the first time acknowledged that the fuel at one of the reactors, Unit 1, likely melted and fell to the bottom of the reactor’s inner pressure vessel.

On Tuesday, Tokyo Electric said that meltdowns likely occurred at units 2 and 3.

It took time for Tokyo Electric to reach that conclusion because it has been gradually retrieving data from the damaged plant and analyzing its findings, Junichi Matsumoto, a senior nuclear official at the company, said at a press conference.

Mr. Matsumoto said data showed that damage to unit 2 began three days after the quake, when its back-up cooling system failed, with most fuel rods eventually melting and collecting at the bottom of the pressure vessel. At unit 3, fuel rods showed signs of damage by the afternoon of March 13.

But many of the details of how the accident unfolded still remain murky. The 18-person IAEA team, which includes experts from the United States, China, Britain and Russia, will interview officials at Tokyo Electric, as well as Japanese nuclear regulators, before drafting a preliminary report next month.

Greeting the team, Banri Kaieda, Japan's trade minister, promised full cooperation.

“We are prepared to disclose all the information we have,” Mr. Kaieda told the team's leader, Mr. Weightman.

Later speaking to reporters, Mr. Weightman expressed some understanding for the severe circumstances workers at the plant faced in the early hours of the crisis.

“In these severe circumstances, when roads, electricity, communications is severely disrupted, how do you manage to have an effective response?” he said.

The Japanese government is conducting separate independent inquiry into the accident. On Tuesday, the government appointed Yotaro Hatamura, known for pioneering a field that studies systematic failures, to head an investigation into the government's response to the disaster.

“We hope that the inquiry is wide-ranging, and treats nothing as off limits, including actions taken by cabinet ministers and the prime minister himself,” said Yoshito Sengoku, deputy chief cabinet secretary.