Japan has asked the World Trade Organisation to step into a row over Seoul's import ban on fish caught in waters near the crippled Fukushima nuclear plant, officials said Tuesday.

Tokyo wants the WTO's Sanitary and Phytosanitary Committee, which deals with food safety, to discuss South Korean rules restricting the import of marine produce from a large area of northern Japan, a fisheries agency official told AFP.

South Korea last month expanded a ban on Japanese fisheries products over fears of contamination from broken reactors at Fukushima, after the plant operator admitted highly toxic water may have made its way into the Pacific Ocean.

The ban takes in products from Fukushima and the seven other prefectures -- Ibaraki, Gunma, Miyagi, Iwate, Tochigi, Chiba and Aomori -- making up the northern half of the main island of Honshu. Gunma and Tochigi are landlocked.

"We will explain in the committee that Japanese aquatic products are under strict safety controls based on international standards, and that the South Korean ban lacks a scientific basis," the official said.

"We've decided to register the issue as part of the agenda to be discussed at the committee after we'd asked South Korea to lift the ban," he said.

Although the committee has no power to impose compulsory orders, "it is better for us to see Seoul voluntarily repeal the ban than to file a suit because it could take years to have a settlement in a formal WTO suit," he said.

South Korea is the only country that has expanded the scope of its ban on Japanese fisheries products after the toxic water leaks of recent months, according to Japanese officials.

Apart from in a small area very close to the plant, scientists say there has been no significant rise in the radiation level of waters in the Pacific.

The spat comes as relations between Seoul and Tokyo continue to be strained amid a rumbling dispute over the sovereignty of a pair of islands and differences over Japan's behaviour in the first half of the 20th century.

Separately, Japan's Nuclear Regulation Authority (NRA) said Monday Tokyo plans to start joint research with the UN's nuclear watchdog and other countries into the effects of the contaminated water leaks.

NRA chief Shunichi Tanaka told a parliamentary committee: "We will seek to ask South Korea and some Southeast Asian countries to participate in the research through the IAEA (International Atomic Energy Agency)," Tanaka said.

IAEA chief Yukiya Amano has said a reliable system conforming to global standards was necessary if Japan was to reassure the international community its marine products are safe.

The agency will send a team from October 14-21 to review Japan's progress on cleaning up areas around the stricken plant.

Fukushima operator TEPCO has long struggled to control the vast amounts of waste water generated by the cooling of runaway reactors in the aftermath of the quake-sparked tsunami of March 2011.

Independent experts say that, ultimately, the utility will have no choice but to dump thousands of tonnes of water -- currently being stored in tanks on the site -- into the ocean, once it has been cleaned of the worst of its radioactive contaminants.