The Japanese arm of the World Wide Fund for Nature urged Yahoo Japan Corp. on Thursday to impose tighter restrictions on sales of ivory items on its auction site, saying it serves as a major platform for the ivory trade and encourages poaching.

Among the nation's other major e-commerce companies, trading of ivory items has disappeared from the Rakuten Inc. website and sharply declined on Mercari Inc.'s community marketplace app after they banned ivory sales, according to a recent survey by WWF Japan.

The survey, conducted between June and July, found ivory items worth ¥37.8 million ($340,000) were sold in four weeks on the Yahoo Auctions site, and that 88 percent of them were processed ivory with no proof of having been legally obtained by the sellers.

"A person charged in an ivory smuggling case in China testified while on trial that they obtained ivory through Yahoo Auctions. It became clear that the site has been used by criminal groups," a WWF Japan official said.

Japan remains one of the world's largest ivory markets and is home to an active, though shrinking, ivory industry, according to WWF Japan. A number of countries, including the United States and China, have closed their domestic markets for ivory.

The Convention on International Trade in Endangered Species of Wild Fauna and Flora, also known as the Washington Convention, prohibits the international trade of ivory in principle to protect African elephants, which are under threat of extinction.