TOKYO—The regulator overseeing Japan's Fukushima Daiichi nuclear complex on Saturday announced a sharp elevation in radioactive contamination had been detected in nearby seawater, furthering signs of distress at a plant where officials had cautioned of radioactive leaks near hobbled reactors the day before.

A spokesman said the spike in radioactive iodine—to 1,250 times the legal limit—didn't pose an immediate threat to human health or the area environment, since the material quickly dissipates in the tides and would become diluted before reaching fish and seaweed.

"Because nobody is engaged in fishery in the evacuation area within a radius of 20 kilometers [from the plant], there will be no immediate impact on people in the area," added Hidehiko Nishiyama, spokesman for Japan's Nuclear and Industrial Safety Agency told reporters at a news conference Saturday morning.

But the news underscores the fact that, for all the progress claimed by officials over the past week, they have a long way to go in bringing Tokyo Electric Power Co.'s reactors under control and understanding exactly what is happening inside the compound.

Saturday's report came a day after efforts to repower key cooling systems at the plant bogged down amid reports of highly radioactive water in puddles at the plant's troubled reactor No. 3. Workers who came into contact with the water on Thursday had to be hospitalized.