Radioactivity levels have soared in the seawater outside the troubled Fukushima Daiichi nuclear plant, safety officials reported Saturday, igniting fresh concerns about the spread of highly radioactive material and the risks involved in completing an already dangerous job.

Samples taken 360 yards offshore from the plant Friday showed radioactive iodine levels 1,250 times the legal safety limit. Earlier in the week, the levels of iodine-131 in the water had been closer to 100 times the limit.

As of Saturday, some signs of progress were evident at the plant: Fresh water was being pumped in to cool the first three nuclear reactors, rather than seawater, which leaves salt deposits that can impair the cooling process. And the lights were turned on in the control room of the second reactor.

But Chief Cabinet Secretary Yukio Edano told reporters at a news conference Saturday that it is difficult to predict when the crisis at the plant might end. He also urged Tokyo Electric Power Co., which operates the plant, to relay information more promptly to the government and improve its transparency.

On Thursday, three workers at the plant sustained severe radiation burns on their legs; two had been wearing ankle boots instead of higher boots that would have offered more protection. Japan’s nuclear agency warned Saturday that Tepco should pay more attention to worker safety.

At the overheated nuclear plant, stricken more than two weeks ago by a 9.0-magnitude earthquake and the resulting tsunami, engineers are awaiting mass shipments of fresh water that can be used to cool the overheated reactors. Two U.S. Navy barges, each carrying 1,100 tons of fresh water, are en route to the plant, and the first of those barges should arrive Monday.

Officials feel a growing pressure to use fresh water rather than seawater for their cooling operations amid concerns that salt deposits left by seawater can corrode the reactors. Water supplied by the U.S. vessels will be pumped into a massive cooling tank at the plant.

Saturday, workers were able to restore lighting in the control room at the unit 2 reactor. Now, only the unit 4 reactor lacks electricity in its control room.

Engineers, meanwhile, turned their attention to cleaning up stagnant, highly contaminated water found in turbine rooms outside the reactors. Pools of the radioactive water have been found at the plant’s units 1 and 3. Similar standing water at units 2 and 4 is being tested for radioactivity.

The unusually high rates of radiation found in the turbine rooms — and now in the ocean — have fueled concerns that water may be seeping from at least one of the reactor cores, leaks that could release longer-lasting and much riskier forms of contamination.

“This is currently one of our largest problems,” said Hidehiko Nishiyama, deputy director general of the government’s Nuclear and Industrial Safety Agency, at a news conference Saturday night.

But government officials said Saturday night that they are not sure whether the primary containment vessels have been breached and are still researching the source of contamination. Analysts say it could be from reactors or from cooling pools where used nuclear rods are stored.

New contamination in the ocean, some nuclear experts say, could also be attributed in part to emissions in the air.

At their highest concentration, near the wastewater outside the plant, the iodine levels in the sea could be dangerous: Half a liter of the water contains the equivalent of the annual approved dosage limit for an adult.

But officials stressed that contaminants would become diluted as currents carry them farther offshore. Fishing has already been banned in the area around the plant. Either way, the spiked radiation levels in the water pose a new concern for Japan’s large fishing industry, with the possibility that other countries could impose bans on imports.

“I don’t believe the levels we detected today would . . . cause a direct problem,” Nishiyama said.

Edano on Saturday announced the appointment of a special adviser to the prime minister, former transportation minister Sumio Mabuchi, to oversee the response to the nuclear crisis.

Intermittent snow and rain covered many of the disaster-affected areas Friday night and throughout the day Saturday, hindering relief efforts and leaving many victims who lack fuel for heating to struggle in the cold.

The government reported that as of Saturday, 10,102 people were listed as dead, 17,053 were missing, 26,646 had been rescued and 246,109 had been displaced from their homes after the March 11 earthquake and the ensuing disasters.

Special correspondent Akiko Yamamoto contributed to this report.