Japan and the United States have agreed in principle to allow U.S. officials to be present as part of the investigators' side when Japanese police question U.S. military personnel suspected of a crime, diplomatic sources said Sunday.

The agreement was reached during two days of official talks by senior working-level officials that ended Thursday at the Pentagon as part of efforts to review the implementation of the Japan-U.S. Status of Forces Agreement, which governs the management and operation of U.S. troops in Japan.

At stake was whether U.S. officials should be allowed to be present at Japanese police questioning of U.S. soldiers suspected of committing serious crimes such as murder and rape.

Japan and the U.S. finally agreed to allow U.S. officials on the pretext they will be present as part of the "investigators' side" and not the "suspect's side."

Unlike the U.S. justice system, Japanese police interrogate criminal suspects without the presence of lawyers. Washington wanted Tokyo's guarantee that suspects' rights will be protected as some human rights groups have raised concerns about the Japanese justice system.

The two countries are expected to reach a formal agreement as early as next month by convening the high-level Japan-U.S. Joint Committee, which specifically deals with SOFA-related issues, the sources said.

But the agreement could lead to controversial arguments within Japan's judicial circles as it allows such a third-party presence only for U.S. soldiers, and may force the government to clarify its reason for allowing the double standard.

The two countries also still have to decide on details such as the qualifications of U.S. officials to be allowed at police interrogations and related procedures, the sources said.