By Kiyoshi Takenaka and Yoko Nishikawa
TOKYO (Reuters) - Environmental group Greenpeace slammed Japan's "inadequate response" to the nuclear disaster at a quake-crippled power plant on Thursday as the plant operator revealed an apparent new leak of radioactive water.
Greenpeace said seaweed had been found with radiation levels 60 times higher than official limits, raising "serious concerns" about long-term risks from contaminated seawater more than two months after the Fukushima-Daiichi plant was destroyed by an earthquake and tsunami.
"In contrast, Japanese authorities claim that radioactivity is being dispersed or diluted and are undertaking only limited marine radiation monitoring," it said.
"Our data show that significant amounts of contamination continue to spread over great distances from the Fukushima nuclear plant," a statement quoted radiation expert Jan Vande Putte as saying.
Greenpeace said the radiation found in seaweed, fish and oysters collected near Fukushima earlier this month exceeded safety limits, suggesting that radioactive water could still be leaking.
One seaweed sample showed readings over 60 times above the limits set by the Japanese government.
"Despite what the authorities are claiming, radioactive hazards are not decreasing through dilution or dispersion of materials, but the radioactivity is instead accumulating in marine life," Vande Putte said.
"The concentration of radioactive iodine we found in seaweed is particularly concerning, as it tells us how far contamination is spreading along the coast, and because several species of seaweed are widely eaten in Japan."
The magnitude 9.0 earthquake on March 11 and the massive tsunami that followed killed about 24,000 people and knocked out the Fukushima plant, triggering the world's worst nuclear accident since Chernobyl in 1986.
The crisis, which has displaced some 80,000 residents from around the plant, prompted a review of Japan's energy policy and growing calls for efforts to step up health monitoring for a crisis now in its 11th week.
The effort to regain control of the Fukushima plant relies on pumping massive quantities of water to cool the three reactors that suffered meltdowns in March and storing the water contaminated in the process in buildings on the site.
Plant operator Tokyo Electric Power said that the water level in a waste storage facility where water was being pumped had unexpectedly dropped, suggesting a new leak of up to 57 tonnes of highly contaminated water.
In early April, the utility dumped about 10,000 tonnes of low-level radioactive water into the ocean, prompting criticism from neighbors China and South Korea.
A poll by the Asahi newspaper published on Thursday showed that 42 percent of Japanese people opposed nuclear power, up from 18 percent before the disaster.
The survey underscored the public's deepening concerns about nuclear safety and criticism of the halting response to the crisis and the incomplete disclosure of key details by both government officials and the utility.
(Additional reporting by Shinichi Saoshiro; Writing by Kevin Krolicki; Editing by Tomasz Janowski and Nick Macfie)