South Korean pharmacists issued an appeal Thursday cautioning against panic over Japan's crisis-hit nuclear plants, as callers flooded drug stores with requests for iodide pills.

Fears over possible radiation has spread across the Internet, prompting Seoul, the closest foreign capital from the stricken nuclear reactors in Fukushima, to launch a crackdown on scaremongering.

Officials on Thursday took pains to stress that westerly winds will blow radiation from Fukushima, some 1,200 kilometres (750 miles) east of Seoul, out into the Pacific.

"Anxiety over radiation exposure is growing in this country following the explosion at Japan's nuclear power plants," the Korea Pharmaceutical Association said in a statement.

"Drug stores are being flooded with calls from people seeking to purchase iodine," it said.

The association said iodide tablets could reduce the risk of thyroid cancer from radiation exposure but could also put users at risk of allergic reactions and problems with thyroid glands.

"As there is little possibility of radioactive dust reaching the country, heavy doses of iodide needed to fend off radiation impact would only bring about health hazards," it added.

Amid deepening fears over radiation, crews on Thursday boycotted ships bound for Japan, Yonhap news agency said, as Seoul started screening travellers from Japan and Japanese food for radiation.

A EU-registered container carrier diverted from its original destination, Yokohama, and returned to South Korea's southern port of Busan, incurring a substantial costs, according to Yonhap.

A luxurious cruiser from Shanghai, the Legend of the Sea, also skipped Fukuoka before making a port call at Busan as passengers were concerned about radiation, the agency said.

President Lee Myung-Bak on Thurday called for a halt to spreading rumours over radiation exposure as police launched a crackdown on text messages and social networking sites.

The Financial Supervisory Service watchdog is investigating the source of a rumour doing the rounds in the markets which exacerbated a stock market plunge on Tuesday.

In South Korea, those who cause confusion by spreading false rumours can be jailed for up to a year.

Aviation companies said Thursday that flights from Tokyo were fully booked as growing numbers of South Koreans joined the exodus from quake-hit Japan.

South Korea's largest airline, Korean Air, said it had "drastically" increased flights linking Tokyo's Narita airport and Seoul's Incheon airport.

It plans to add three to five flights to its normal four flights per day, to carry an additional 5,000 passengers between Wednesday and Sunday, a Korean Air spokesman said.

Asiana Airlines, the second largest South Korean airline, said it had added one flight a day to its regular four from Narita to Incheon.

It cancelled all flights to and from Fukushima for fears over radiation fallout after it closed the routes to the tsunami-hit Sendai City and Ibaraki Prefecture.

The Korea Football Association said Thursday that Montenegro had called off a football friendly with South Korea scheduled for next week because of concerns over feared radiation fallout.

"Players, their relatives and clubs opposed the visit to Seoul because of the quake and radiation leaks," it said in a statement.