JAPAN'S nuclear crisis has sparked panic buying of iodine pills, with online bids exceeding $500 for a single packet, but health experts hosed down the hysteria and warned the pills are of limited use.

As fresh blasts rocked a stricken atomic plant on Japan's east coast, and crews worked frantically to cool reactors that emitted dangerous levels of radiation near the facility, jitters spread to Tokyo and beyond.

US-based firms selling potassium iodide, a radiation sickness preventative, completely ran out of stock and pharmacies across the country's Pacific-facing West Coast had a rush on the over-the-counter pills.

"We are quite slammed with orders, but we are working as fast as we can to get orders out," said NukePills.com, which had sold out of iodine tablets and was fast exhausting oral liquid supplies.

"We are experiencing delays in shipping due to the Japan nuclear crisis. A delay in shipping may be a week or more."

Potassium iodide is a salt used to saturate the thyroid gland to block the uptake of radioactive iodine, a highly carcinogenic substance that can leak from nuclear reactors in an accident.

Another major supplier, Anbex, said it was also out of stock and didn't expect new orders until April 18.

One packet of 14 pills had attracted bids of up to $US540 ($A535.87) at online auction house eBay and talk about radiation poisoning was so feverish on Twitter and other forums that the World Health Organisation issued a statement urging calm.

"Consult your #doctor before taking #iodine pills. Do not self-medicate!" the WHO said on its Twitter page.

Iodine pills are "not radiation antidotes" and offer no protection against radioactive elements such as caesium, the UN's health agency said, stressing they also carried health risks for some people, including pregnant women.

The WHO also cautioned against drinking or applying iodine liquid, commonly used as an antiseptic, after a rush on the wound cleaner in Asian countries, where iodine is typically only available in hospitals or by prescription.

"It is crazy, people have been reading about the situation in Japan and they are demanding iodine tablets but most pharmacies don't stock the tablets," said Kuala Lumpur pharmacist Paul Ho.

"There have also been text messages and emails going round that you can use the iodine antiseptic solution, which you place around your neck, to help cut down on radiation absorption," he added.

"I don't know if its true but we have run out of all our iodine antiseptic solution at the moment."

The text message, also circulating in China, Hong Kong and the Philippines, is billed as a "newsflash" from a major news organisation and urges Asian residents to "take precautions" including sheltering indoors and swabbing the thyroid region of the neck with iodine.

Malaysia's Health Minister Liow Tiong Lai dismissed the purported warning as "nonsense", saying there was "no need to apply such solutions to the neck and private parts."

"People must not panic. The health ministry is keeping very close tabs on the situation," he told AFP.

The assurances were echoed in Taiwan, where officials were preparing to hand out 100,000 boxes of iodine tablets to residents near two nuclear plants in New Taipei city, and also in Manila.

"Let me be very clear, we don't see the necessity for that," said Philippines Health Secretary Enrique Ona.

"We know where we can get it if necessary. But we are not going to order it yet."

Stephen Tsui, a biomedical expert from the Chinese University of Hong Kong, described the risk of contamination outside of Japan as "low" but said "all countries could be affected" in the region if the Fukushima plant had a total meltdown.

Tens of thousands have already been evacuated from a zone within a radius of 20 kilometres of the 40-year-old plant, where authorities said radiation levels reached dangerous levels today.

Higher than normal radiation was also detected in Tokyo, but a city official said it was not considered at a level harmful to human health. Both levels fell later in the day.