Customers at a supermarket on Thursday in Wuhan, capital of Hubei province, are not allowed to buy more than five packs of salt at one time. [Photo / China Daily]

'Rumors are harmful'

The Chinese government also weighed in Thursday, with Foreign Ministry spokeswoman Jiang Yu saying, "I do not see any necessity to panic."

In its notice, the Development and Reform Commission also urged local authorities to take "immediate action to monitor the market prices and resolutely crack down on illegal acts, including spreading rumors to deceive the public".

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Michael O'Leary, WHO's representative in China, called on governments and individuals to "take steps to halt these rumors, which are harmful to public morale".

O'Leary said WHO "would like to assure governments and members of the public that there is no evidence at this time of any significant international spread from the nuclear site".

Su Xu, a researcher with the Chinese Center for Disease Control and Prevention, said that iodized salt would do little to help protect against radiation and warned that taking excessive amounts of iodine was harmful.

"Iodine drugs should be used only under the medical supervision of doctors and medicine specialists," he said.

Others worry the phenomenon showed just how far people's trust in official information has diminished.

AP, Reuters and Cao Yin contributed to this story.