Japan cut the outlook for its economy on Wednesday for the first time in six months, saying last month's devastating quake and tsunami would hurt growth, with no sign yet when the nuclear crisis they triggered might be brought under control.

New data shows much more radiation leaked from the Fukushima Daiichi nuclear power plant in the early days of the crisis than first thought, though the U.S. nuclear safety regulator said the crisis appeared to be "static" as engineers at the Fukushima nuclear plant struggle to cool overheating fuel rods.

After recent fears of possible contamination in the region, neighbouring China said that the impact there had been small, noting the amount of radiation was about one percent of what it had experienced from the world's worst nuclear disaster at Chernobyl.

The total cost of the March 11 triple disaster has been estimated at $300 billion, making it the world's most costly natural disaster.

"The effects of the earthquake will be temporary. It will cause various indirect damage such as dampening consumer sentiment but the economy will pick up toward the end of this year. That's what many economists are thinking," Japanese Economics Minister Kaoru Yosano said.

The new economic assessment for a country that has been mired in recession for most of the past 15 years came after another relatively strong quake rocked northeastern Japan on Wednesday. But there were no reports of any damage to a region already devastated by last month's tremor and tsunami.

Plant operator Tokyo Electric Power Co (TEPCO) appears to be no closer to restoring cooling systems that are critical to lowering the temperature of overheated nuclear fuel rods in the damaged reactors.

The company has begun looking into how it will store and transport the spent fuel from the reactors, though work cannot start until they are in cold shutdown, TEPCO official Mitsuo Matsumoto told reporters.

It is expected to take months before the damaged reactors will have cooled down. Some officials have speculated that the authorities may have to entomb the plant if the crisis drags on too long, the solution that was eventually used to close off Chernobyl.

On Tuesday, Japan's science ministry said small amounts of strontium, one of the most harmful radioactive elements, had been found in soil near Fukushima Daiichi.

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.