Japan has asked the World Trade Organisation to step into a row over Seoul's ban on fish caught in waters near the crippled Fukushima nuclear plant. Tokyo wants the WTO's food safety panel to discuss South Korean rules restricting the import of marine produce from a large area of northern Japan. 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. "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," a fisheries agency 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." Although the committee had 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", the fisheries official 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 a small area close to the plant, scientists say there has been no significant rise in the radiation level of Pacific waters. 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. Separately, Japan's Nuclear Regulation Authority said on Monday that Tokyo planned to start joint research with the UN's nuclear watchdog and other countries into the effects of the contaminated water leaks. Authority 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]." 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 were safe. 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 - now being stored in tanks on the site - into the ocean, once it has been cleaned of the worst of its radioactive contaminants.