Tokyo ? Japan has warned leaders of the House of Representatives that serious, long-term damage to Japanese-U.S. relations is likely if the House passes a resolution demanding an official apology from Japan for its wartime policy of forcing women to become sex slaves for Japanese soldiers.

In an unusually blunt letter sent to five House leaders, including Speaker Nancy Pelosi, D-Calif., Japanese Ambassador Ryozo Kato said passage of the resolution “will almost certainly have lasting and harmful effects on the deep friendship, close trust and wide-ranging cooperation our two nations now enjoy.”

The ambassador said that since 1993 Japan has repeatedly and officially apologized for its harsh treatment of “comfort women,” the term used for the estimated 50,000 to 200,000 Asian women forced by the Japanese government into brothels before and during World War II.

His June 22 letter, obtained by The Washington Post, also suggests that Japan may reconsider its role as one of the few loyal supporters of U.S. policy in Iraq, where it is the second-largest donor for rebuilding, after the United States.