Japanese authorities issued contradictory assessments Tuesday about the severity of the crisis at the Fukushima Daiichi nuclear plant, with Prime Minister Naoto Kan saying that the situation is improving “step by step” and a senior official from the operating power company warning of further mass radiation releases.

The mixed signals provided a backdrop to a decision earlier in the day by Japan’s nuclear agency to raise the severity rating of the nuclear crisis at Daiichi from Level 5 on the International Nuclear and Radiological Event Scale (INES), indicating an “accident with off-site risk,” to Level 7, a “major accident” on a par with the 1986 Chernobyl disaster.

The reassessment came amid reports from the International Atomic Energy Agency that the plant is showing “early signs of recovery” but remains in critical condition. The upgraded severity reading does not reflect a recent deterioration at the plant. Rather, it suggests Japan’s evolving understanding of the damage that occurred there one month ago and the contamination that has been leaking since.

The most severe radiation leak occurred in the 48 hours after a March 15 hydrogen explosion at the unit 2 reactor, Japan’s Nuclear and Industrial Safety Agency said. Nuclear experts said Tuesday that the government waited too long to upgrade the INES rating, given the vast amounts of radiation being released.

“Monitoring data available shows that, in my view, the government probably knew around March 16, 17 or 18 that it would reach Level 7,” said Hironobu Unesaki, a professor at the Kyoto University Research Reactor Institute. “Their response has been extremely regrettable. The government is being very careful not to cause unnecessary panic, but they are being too cautious.”

At a news conference in Tokyo to announce the new INES rating, Hidehiko Nishiyama, deputy chief of the Nuclear and Industrial Safety Agency, emphasized that radiation released from Fukushima Daiichi amounted to less than one-tenth the total released from Chernobyl. Later in the day, Kan said radiation levels around the plant were steadily dropping.

Those calming messages, though, were undercut at another separate news conference, during which an official from Tokyo Electric Power Co., the plant’s operator, said the company’s concern was “that the amount of leakage could eventually reach that of Chernobyl or exceed it.”

That stark assessment reinforced apprehension that this nuclear emergency will cause greater problems than those first predicted by the government, which has played down long-term safety concerns and only Monday expanded its mandated 12-mile-radius evacuation zone.

On Tuesday, Kan defended the government’s response, saying that its decision to recategorize Fukushima as a Level 7 event “does not show that we delayed or underestimated the nuclear situation.”

A Level 7 accident, according to the INES scale, is typified by a “major release of radioactive material with widespread health and environmental effects.” The scale was established by the IAEA about 20 years ago, but its guidelines leave much room for interpretation, nuclear experts say.

The new rating follows a recent report by Japan’s Nuclear Safety Commission estimating the total amount of radiation released by the facility. Since the March 11 earthquake and tsunami, damaged reactors and spent fuel pools at Fukushima have released 370,000 terabecquerels of radiation into the environment, the commission reported. That’s 14 times less radioactivity than was released at Chernobyl. A becquerel is the measure of the radioactive decay of one unstable atom; a terabecquerel is a trillion becquerels.

Denis Flory, deputy director general of the International Atomic Energy Agency, said the INES rating does not dictate responses to accidents. A higher ranking from the Japanese government early in the incident would have made no difference in the government’s actions, which have included an ever-widening evacuation zone and close monitoring of food, milk and water, he said.

“The fact it was a 5 until now only means that [the Japanese government] had not evaluated the quantity of radioactivity released,” Flory said.

But a nuclear watchdog disagrees. Assigning a higher rating earlier may have drawn more international assistance while forcing the Japanese to expand the evacuation zone, said David Albright, president of the Institute for Science and International Security in Washington.

“They should have made a higher ranking,” he said.

Despite improving radiation readings, the plant still faces numerous risks. Thousands of tons of contaminated water have flooded key buildings adjacent to the reactors. Nitrogen gas is being injected into one unit to prevent another explosion.

In the meantime, the plant faces the constant threat of aftershocks. A 6.2-magnitude temblor Tuesday caused a brief fire, although it did not interrupt the process used to cool hot fuel rods. But a day earlier, a 6.6-magnitude quake with an epicenter just 42 miles from the plant caused a 50-minute interruption.

“We are not at the stage yet where we can let our guard down,” Kan said.

Although the Fukushima crisis now stands alongside Chernobyl on the INES scale, experts note several important distinctions. No deaths have resulted from radiation leaked at Fukushima. At Chernobyl, a single reactor exploded, killing dozens of workers, and a massive cloud of cesium, plutonium and strontium spread across Belarus, Ukraine, Russia and Europe, causing a still-undetermined number of deaths.

At Fukushima, workers have struggled to stabilize seven reactors and spent fuel pools, but the radioactive releases have come as byproduct of emergency cooling efforts, with the venting of radioactive steam and the leaking of contaminated water.

Technically, Japan’s reassessed rating applies to only three of Fukushima’s six units — Nos. 1, 2 and 3, which have all sustained core damage. On March 18, each of those units had been initially given a Level 5 rating. At the same time, Japan gave a Level 3 rating to unit 4; that remains unchanged. The IAEA cautioned that Japan could still change its ratings as more information becomes available.

The latest reevaluation underscores the difficulty of measuring the amount of contamination released and the danger it poses. Rather than creating a hard “no-go” circle around the plant, the government has instead singled out five towns between 12 and 19 miles from the plant for mandatory evacuation. Residents there should leave within a month, the government said.

Special correspondent Akiko Yamamoto in Tokyo and staff writer Brian Vastag in Washington contributed to this report.