The U.S. and Japan still have grave disagreements that must be resolved for the Trans-Pacific Partnership (TPP) to be a success.

Japan wants to preserve high tariffs in five key agricultural categories such as rice and beef. The U.S.-led TPP insists on eliminating those trade barriers entirely.

Trade representatives for Japan and the United States in Tokyo this week to try to bridge those differences before the four-day-long ministerial conference in Singapore that will start on Saturday. Despite their efforts, no agreement has been reached yet.

Japanese Economy Minister Akira Amari said that there is still a wide gulf of disagreement between the two sides, but he "will strive to diminish that as much as possible."

Japanese trade affiliates are aware that it is difficult to reach an agreement unless the negotiation is settled in the Ministerial Conference. The U.S. and Japan are the two biggest economies involved in the 12-nation TPP.