The operation to cool the reactors at the Fukushima Daiichi nuclear power plant has suffered a setback after smoke and vapour was seen rising from two reactors, as anxiety grew over the safety of food produced in the area.

Water in a pool for storing spent fuel was reaching boiling point, raising the possibility that spent fuel rods could be exposed in turn releasing further radiation, said an official from the nuclear safety agency.

"We cannot leave this alone and we must take care of it as quickly as possible," said the official, Hidehiko Nishiyama.

Days after Japanese authorities reported abnormal levels of radiation in milk, some vegetables and tap water, the plant's operator, Tokyo Electric Power (Tepco) said high levels of radioactivity had been found in seawater near the facility, raising fears that seafood has also been contaminated.

The power company said seawater samples contained levels of radioactive iodine 126.7 times the allowed limit, and caesium 24.8 times over. The firm said the quantities posed no immediate threat to health.

"You would have to drink [the seawater] for a whole year just in order to accumulate one millisievert [of radiation]," a Tepco official said. Background radiation from substances in the air and soil generally emit between one and 10 mSv a year.

The source of the contamination has yet to be established, but officials believe it probably came from the tonnes of seawater that have been sprayed over overheating reactors and fuel rod pools in recent days.

The Fukushima prefectural government said no marine products from the region had been distributed after the earthquake. Any evidence that contamination has spread to seafood would add further misery to the region's food producers.

About 6,000 residents of a village near the plant have been told not to drink tap water, while a ban has been imposed on shipments of milk, spinach, and kakina – a leafy vegetable – produced in the area.

The government said it had no plans to extend a 20km (12-mile) evacuation zone around the nuclear plant, despite elevated radiation readings outside the area. "At the moment, there is no need to expand the evacuation area," the government's chief spokesman, Yukio Edano, told a briefing.

The government's latest readings, taken 10km outside the evacuation zone, show radiation levels of 110 microsieverts per hour, higher than normal background levels but well below those deemed a risk to health.

The Kyodo news agency quoted International Atomic Energy Agency data showing that radiation levels 1,600 times higher than normal had been detected in a now-evacuated residential area near the crippled atomic plant.

According to agency inspectors, radiation levels of 161 microsieverts per hour were detected in the town of Namie, about 20km away.

Amid growing concerns over food safety, Japanese authorities are reportedly considering a wider ban on the shipment of certain foodstuffs from the affected region. "They're going to have to take some decisions quickly in Japan to shut down and stop food being used completely from zones which they feel might be affected," Gregory Hartl, a World Health Organisation spokesman, told Associated Press in Geneva.

The WHO said the long-term effects on health from contaminated food was of greater concern that the spread of radioactive particles in the air. "A week ago we were more concerned about purely the radiation leakages and possible explosion of the nuclear facility itself, but now other issues are getting more attention, including the food safety issue," Hartl said.

"Repeated consumption of certain products is going to intensify risks, as opposed to radiation in the air that happens once, and then the first time it rains there's no longer radiation in the air."

The WHO said the spread of radiation into the food supply was more serious than it had first thought, although it added that no tainted products had reached overseas markets.

Japan is an exporter of seafood, fruit and vegetables, and dairy products, with Hong Kong, China and the US its biggest markets. China, Taiwan and South Korea said they would tighten screening of Japanese imports.

Workers at the Fukushima plant have attached power cables to all six reactors and started pumping water into one of them in an attempt to prevent overheating fuel rods from reaching the potentially catastrophic meltdown phase.

"There are signs of light that we are getting out of this crisis," the prime minister, Naoto Kan, was quoted as saying.

But that optimism was tempered by the sight of what appeared to be steam rising from the No 2 reactor and smoky haze above the No 3 reactor. The latter has given particular cause for concern because it contains plutonium-uranium mixed oxide fuel – or MOX – and would release highly toxic plutonium in the event of a meltdown.

Tepco officials said the emissions had temporarily halted work to cool off the reactors.

The death toll from the earthquake and tsunami continued to rise on Tuesday, as more bodies were retrieved from the vast stretch of coastline hit by the tsunami. Police said 8,928 people had been confirmed dead and a further 12,664 were missing. Various estimates have put the current death toll at nearer 18,000.