Operator Tokyo Electric Power has poured massive amounts of water on four of the reactors at the plant to cool the fuel they contain, but struggled to keep the radioactive water from leaking out to the sea.
Environmental critics have also raised worries about contaminated water seeping into the water table.
Greenpeace said that 10 of the 22 seaweed samples it had collected at sea near the Fukushima Daiichi nuclear plant had shown radiation levels as much as five times the standard set by Japan for food.
"Radioactive contamination is accumulating in the marine ecosystem that provides Japan with a quarter of its seafood, yet the authorities are still doing very little to protect public health," Ike Teuling, Greenpeace radiation expert, said in a statement.
On Wednesday, Tepco sealed a leak of contaminated water found near the No. 3 reactor that may have seeped into the Pacific Ocean from the coastal plant. A previous ocean leak sparked international concern about the impact of the disaster on the environment.
Radiation in food and other substances is measured in Becquerel. The limit set for food by Japan is 2,000 Becquerel (Bq) per kg for radioactive Iodine-131 and 500 Bq/kg for radioactive cesium.
Greenpeace said 10 of its seaweed samples had shown radiation levels over 10,000 Bq/kg. Some types of seaweed are a staple of the Japanese diet.
Goshi Hosono, a special adviser to Prime Minister Naoto Kan on the nuclear crisis, said the government would look into the finding by Greenpeace.
"I don't want to ignore this and the government will do its own follow-up study as needed," Hosono told reporters.
Separately, government officials in Tokyo said radiation levels in sewage had spiked in late March. The data was released this week in conjunction with a new government standard intended to contain the spread of radiation in sewage.
Combined radiation levels of cesium and others in waste burned at the sewage treatment plant in Tokyo spiked to 170,000 Bq/kg in the immediate wake of the Fukushima nuclear crisis, officials said on Friday.
The radiation level was measured on March 25, just over two weeks after the earthquake.
Akiko Matsumoto, spokeswoman for the Tokyo Bureau of Sewage said the radiation figure is a composite of cesium and iodine levels.
The Japanese government did not set a guideline for radioactive material in sewage until Thursday, when they announced that any solid waste with a cesium level of 100,000 Bq/kg or above should be incinerated and then sealed in a container.
Matsumoto said the bureau did not know the reason behind the sudden jump in radiation levels. "One theory is that the radiation from Fukushima was carried by rain," she said.
The overall radiation level in Tokyo sewage had dropped to 16,000 Bq/kg by April 28.