Tokyo risks falling down on its pledge to stage an environmentally sustainable 2020 Olympics due to the import of illegally felled timber from Southeast Asia, activist group Global Witness said.

Some of Japan's biggest construction companies buy plywood from Borneo, where the rain forest is being logged inside a national park and on land set aside for native tribes, the U.K.-based group said in a report Monday.

Japan is the world's biggest importer of plywood from tropical rain forests, Global Witness said. The nation vowed to make the environment a top priority in its winning bid for the Olympics. Two of its main suppliers, Malaysia's Samling Group and Shin Yang Group, based in Sarawak province, have faced allegations of illegal and unsustainable logging, Global Witness said.

"The Japanese government has been aggressively selling the green credentials of the Tokyo Games, but the secret of Japan's construction industry — its reliance on rain forest timber logged destructively in Sarawak — is being left out of the rhetoric," Global Witness said.

Neither of the Sarawak-based companies responded to Global Witness' request for comment, although Samling has denied allegations of illegal logging in the past and did so again in comments to reporters. An official at Shin Yang said Monday the company is preparing a response.

Global Witness said construction companies Taisei Corp., Shimizu Corp., and Kajima Corp. "have indicated they are taking steps to review their sourcing practices and look into sustainable alternatives to tropical plywood."

According to the committee organizing the 2020 Games, the event is still in an early stage of preparation and no construction work has commenced. The organizers remain "fully committed" to a sustainable Olympics and are currently forming a policy with strict criteria for all products and services, the committee said in an emailed answer to questions.

Alongside 11 permanent venues and other facilities for the games, the organizers have said they would carve out 433 hectares of new green areas in Tokyo and plant 1 million roadside trees by 2020.

The Tokyo Metropolitan Government, which is in charge of building permanent facilities for the games, said it has asked for construction work to use "environmentally friendly plywood" to reduce imports of tropical lumber, according to Chihiro Sai, a spokeswoman for the metropolitan games preparation division.

Construction firms typically use sheets of plywood as molds when pouring concrete, after which they are discarded.

A Shimizu spokesman said Friday he could not immediately comment on the Global Witness report and was not immediately available to speak Monday. Spokeswomen for Taisei and Kajima declined comment.

The conservation area allegedly being logged, known as the Heart of Borneo, has more than 22 million hectares of rain forest, which are overseen by Brunei, Malaysia and Indonesia.

Samling spokeswoman Ellis Loke told reporters in an email that the company is "not involved in any illegal logging activities and we are not involved with the one allegedly being carried out in a national park."

"As for native communities, the Malaysian government allows local indigenous communities to continue living on or near concessions," Loke said. "We have always taken care of these communities and fully respect their traditions and customary laws. We also recognize native land rights, and acknowledge that a few communities have not fully understood the parameters of native land rights."

According to the Global Witness report, the efforts by Japanese companies to deliver environmentally sustainable construction includes Taisei reporting "on the amount of tropical plywood it uses for concrete form-work as part of an effort to increase the use of alternative materials."

Kajima said it is trying to change its sourcing practices and is also looking into alternatives, according to Global Witness, while Shimizu said it is "raising the issue of illegal logging in Sarawak with its procurement office and contractors, and is reviewing the use of plywood in a construction project named in the report."