Contamination of seawater around the Fukushima Daiichi nuclear plant in northeastern Japan has soared to 1,250 times the normal figure, the Japanese Nuclear and Industrial Safety Agency said Saturday.

The concentration of radioactive iodine-131, recorded Friday morning was the highest level since the agency began sampling seawater this week near the plant 220 kilometres north of Tokyo.

The agency said there should be no immediate effect on human health, and Yukio Edano, Japan's chief cabinet secretary, said marine life hasn't been noticeably affected either, as far as the government knows.

Monitoring is to be strengthened, he said.

The nuclear crisis is bound to go on for "a long time," Edano said. "We seem to be keeping the situation from turning worse. But we still cannot be optimistic."

Fears of breach

Reporter Steve Futterman told CBC News that radioactive iodine-131 dissipates quickly, according to nuclear experts, but high contamination readings are fuelling fears a reactor core is leaking.

"No one knows for sure if there's been a breach, but you have these extremely high rates — that's what make people believe there could be a breach."

Radiation has been seeping from the plant since a magnitude-9 earthquake and a tsunami on March 11 knocked out its cooling systems. The contamination has made its way into milk and vegetables, including broccoli, cauliflower and turnips.

Tap water in Tokyo and several other areas of Japan has shown higher-than-normal levels of radiation. Tokyo readings were at 1.2 times higher than the government safety limit for infants, who are particularly vulnerable to radioactive iodine.

Water safe for infants

But levels have fallen since peaking Wednesday, and city officials in Tokyo said Saturday that tap water was now safe for infants to drink.

Meanwhile, U.S. naval barges loaded with fresh water sped toward the Fukushima plant, where workers are trying to stem the rise in radioactivity and remove contaminated water from the facility.

They have been using seawater in to stabilize the overheating reactors, but this raised fears about the corrosive nature of salt in the water.

Plant operator Tokyo Electric Power Co. switched to freshwater, after an urgent request from the U.S. government. The water on the barges should be ready for injecting into the plant early next week.

Nuclear safety officials suspect a breach in one or more of the plant's units, possibly a crack or hole in the stainless steel chamber around a reactor core or the concrete wall surrounding a pool where spent fuel rods are stored.

Suspicions were aroused when two workers suffered skin burns after encountering water that was 10,000 times more radioactive than levels normally found in the units, Japanese safety officials said.

Such a breach could mean a much larger release of radioactive contaminants than had been thought.