The operator of the Fukushima No. 1 nuclear power plant in northeastern Japan has detected radioactive iodine 1,250.8 times above legal limit in sea near the plant, local media reported Saturday.

The level jumped to its highest so far in the survey begun this week, Tokyo Electric Power Co said.

According to the government's Nuclear and Industrial Safety Agency, the readings were taken Friday morning about 330 meters south of the plant, near the drain outlets of the reactors at the utility.

Radioactive materials "will significantly dilute" by the time when they are consumed by marine species, Kyodo News reported, citing the agency.

The agency added that the radiation will not have a significant impact on fishery products because fishing is not conducted in the area within 20 km of the plant.

The Japanese authorities have been stepping up efforts to fix the nuclear power plant crippled by devastating March 11 earthquake and tsunami.