Activist groups called for an independent investigation into the use of wood possibly originating from Malaysian forests for the construction of Tokyo's Olympic stadium, saying it is a "significant breach" of the city's commitment to a sustainable 2020 Games.

The seven groups, which include Rainforest Action Network and Friends of the Earth Japan, said in a statement Thursday that tropical plywood being used to mold concrete for the stadium had markings that appear to belong to Shin Yang, a Malaysian group of businesses. The groups said that Shin Yang is one of the "Big Six" logging companies in Sarawak state on the island of Borneo, a place where they say illegal logging is widespread and the destruction of forests "extreme."

Calls and emails to Gary Tan, the general manager of Shin Yang, were not immediately returned. The company says on its website that it follows practices aimed at protecting the environment and ensuring sustainable forestry.