TOKYO — Japanese officials struggled through the day on Tuesday to explain why it had taken them a month to disclose large-scale releases of radioactive material in mid-March at a crippled nuclear power plant, as the government and an electric utility disagreed on the extent of continuing problems there.

The government announced Tuesday morning that it had raised its rating of the severity of the accident at the Fukushima Daiichi Nuclear Power Station to 7, the worst on an international scale, from 5. Officials said that the reactor had released one-tenth as much radioactive material as the Chernobyl accident in 1986, but still qualified as a 7 according to a complex formula devised by the International Atomic Energy Agency.

Japan’s new assessment was based largely on computer models showing very heavy emissions of radioactive iodine and cesium from March 14 to 16, just after the earthquake and tsunami rendered the plant’s emergency cooling system inoperative. The nearly monthlong delay in acknowledging the extent of these emissions is a fresh example of confused data and analysis from the Japanese, and put the authorities on the defensive about whether they have delayed or blocked the release of information to avoid alarming the public.

Seiji Shiroya, a commissioner of Japan’s Nuclear Safety Commission, an independent government panel that oversees the country’s nuclear industry, said that the government had delayed issuing data on the extent of the radiation releases because of concern that the margins of error had been large in initial computer models. But he also suggested a public policy reason for having kept quiet.