Japanese Prime Minister Naoto Kan told a news conference that Tokyo Electric Power would soon come up with an outlook for when it would get the nuclear crisis at its Fukushima Daiichi plant under control.

Kan also said that radiation leaks were declining at the plant, 240 km (150 miles) northeast of Tokyo.

Japan earlier on Tuesday designated the March 11 accident at the quake and tsunami-damaged plant as a level 7, on par with the world's worst nuclear calamity, Chernobyl.

Officials said it had taken time to measure radiation from the Fukushima Daiichi facility 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 recognised scale did not mean the situation had suddenly become more critical.

"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 Prime Minister Naoto Kan's office.