Delegates from around the world paid tribute Wednesday to the hundreds of Japanese who were killed by decades-long mercury dumping as they gathered at the site of the country's worst industrial poisoning.

Representatives from 140 countries and territories laid flowers at a monument to the dead at Minamata in southern Japan, before the signing on Thursday of an international treaty to control the use of the toxic metal.

The Minamata Convention on Mercury is named after the Japanese city where tens of thousands were made ill -- around 2,000 of whom have since died -- by eating fish and shellfish taken from waters polluted by discharge from a local factory.

The scandal first came to light in the 1950s, but it was not until more than 50 years later that the state fully recognised the extent of the problem.

Mercury poisoning affects the body's immune system and the development of the brain and nervous system, posing the greatest risk to foetuses and infants.

The substance, also known as quicksilver, is found in products ranging from electrical switches, thermometers and light bulbs to amalgam dental fillings.

The treaty to be signed Thursday sets a phase-out date of 2020 for a long list of products -- including mercury thermometers -- while the text gives governments 15 years to end all mercury mining.

But environmental groups say it stops short of addressing the use of mercury in artisanal and small-scale gold mining, which directly threatens the health of miners including child labourers in developing countries.

They also warn of health risks from eating the mercury-polluted meat of whales and dolphins, which sometimes feature on the diets of coastal communities in Japan and elsewhere.

Because of their position near the top of the food chain, dolphins and whales can consume a large quantity of mercury from their prey.

"For far too long, coastal communities around the world have been allowed to consume the mercury-contaminated meat of whales, dolphins and porpoises, many in ignorance of the risks involved," said the UK-and US-based Environmental Investigation Agency.

"Now signatories to the new treaty must make communities in places as far afield as Japan and the Faroe Islands properly aware of the very serious risks to human health that come from eating the meat of toothed cetaceans," it said in a statement.

The dolphin-hunting town of Taiji in western Japan, made infamous by the Oscar-winning film "The Cove", regularly draws international criticism for its bloody slaughter of the creatures, which are then used for meat.

But the town, which this month announced plans for a marine mammal park where people could swim or kayak with dolphins and then eat their meat, defends the practice as part of a 400-year-old whaling and culinary tradition.

Still, the Japanese government recognises the possible risk of eating dolphin meat

The health ministry advises pregnant women not to eat more than one 80-gram (2.8 ounce) serving of short-finned pilot whale meat every two weeks. The same amount of bottlenose dolphin meat is the recommended limit every two months.