TOKYO – Japan put its nuclear calamity on a par with the world's worst nuclear disaster, Chernobyl, on Tuesday after new data showed that more radiation leaked from its earthquake-crippled power plant in the early days of the crisis than first thought.

Japanese officials said it had taken time to measure radiation from the plant after it was smashed by March 11's massive quake and tsunami, and the upgrade in its severity rating to the highest level on a globally recognized scale did not mean the situation had suddenly become more critical.

"The situation at the Fukushima Daiichi plant is slowly stabilizing, step by step, and the emission of radioactive substances is on a declining trend," Prime Minister Naoto Kan told a press briefing on Tuesday.

He also called on opposition parties, whose help he needs to pass bills in a divided parliament, to take part in drafting reconstruction plans from an early stage.

"Our preparations for how to measure (the radiation leakage) when such a tsunami and earthquake occurred were insufficient and, as a result, we were late in disseminating information internationally," said a senior official in Kan's office.

Hidehiko Nishiyama, a deputy director-general of the Nuclear and Industrial Safety Agency (NISA), said the decision to raise the severity of the incident from level 5 to 7 -- the same as the Chernobyl disaster in Ukraine in 1986 -- was based on cumulative quantities of radiation released.

As another major aftershock rattled the earthquake-ravaged east of the country a fire broke out at the plant, but engineers later extinguished the blaze.

However, the operator of the stricken facility appears to be no closer to restoring cooling systems at the reactors, critical to lowering the temperature of overheated nuclear fuel rods.

No radiation-linked deaths have been reported since the earthquake struck, and only 21 plant workers have been affected by minor radiation sickness, according to Chief Cabinet Secretary Yukio Edano.

"Although the level has been raised to 7 today, it doesn't mean the situation today is worse than it was yesterday, it means the event as a whole is worse than previously thought," said nuclear expert John Price, a former member of the Safety Policy Unit at the UK's National Nuclear Corporation.