United States diplomatic cables published by Wikileaks show that Japanese and US officials have discussed ways of reining in theSea Shepherd anti-whaling group.
The group has been successful in hampering Japanese whalers during the annual Antarctic hunt.
The cables reveal that the US envoy to the International Whaling Commission, Monica Medina, held talks with the head of Japan's fisheries agency, Katsuhiro Machida, in late 2009.
The two sides discussed the possibility of revoking the tax-exempt status of the Sea Shepherd Conservation Society which is based in the United States.
In the cables sent from the US embassy in Tokyo, the Japanese were said to have appreciated the American idea to remove the exemption.
The documents reveal Ms Medina believed the US government could demonstrate the group did not deserve tax-exempt status because of what were perceived as its aggressive and harmful actions.
This week the organisation that runs Japan's scientific whaling programme called on Australia to restrain Sea Shepherd activists from harassing its fleet in the Antarctic ocean.