The Bush administration is trying to enlist Tokyo's support for its controversial decision to back India's civilian nuclear power development.
U.S. President George W. Bush will broach the issue with Prime Minister Junichiro Koizumi at their June 29 summit meeting in Washington, sources said.
Koizumi is among Bush's staunchest allies, but it is not clear how far he will go to express support for the U.S.-India agreement.
Some officials are concerned the deal would further undermine the Nuclear Nonproliferation Treaty (NPT). Others say that Japan, as the only nation to have had atomic weapons used against it, should not give its backing.
India has conducted nuclear weapons tests, but it has not signed the NPT.
U.S. officials argue that the nonproliferation structure has been strengthened because India is allowing the International Atomic Energy Agency (IAEA) to inspect its civilian nuclear facilities. That agreement was a precondition to India's purchase of nuclear technology and fuel from the United States.
However, the U.S.-India deal does not cover inspections of India's military facilities, leading to concerns that nuclear weapons would effectively be placed outside of the international arms control structure.
Japanese government sources said U.S. officials asked for an expression of support soon after the deal was signed on March 2.
Japan has not stated its official position, but government officials are considering issuing a statement of "basic understanding" of the agreement.
Among points in favor of Japan expressing its support are that the deal enables inspections of India's civilian nuclear facilities, thus strengthening the nonproliferation structure. The agreement would boost India's economic growth. Britain and France have already expressed their support.
However, some government officials are concerned about the lack of guarantees that IAEA inspectors will be able to carry out their inspections in India.
In addition, they worry that nuclear technology developed in India's civilian sector could be converted to military purposes.
Some officials think Japan should delay any expression of support until the July Group of Eight meeting in St. Petersburg, Russia, which Britain and France will also attend.