TOKYO — In a belated acknowledgment of the severity of Japan’s nuclear disaster, the Tokyo Electric Power Company said Tuesday that three of the stricken Fukushima Daiichi plant’s reactors most likely suffered fuel meltdowns in the early days of the crisis.

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 on Tuesday, a team 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, Banri Kaieda.