Outreach to the so-called "Global South", shorthand for some low- and middle-income countries including India, has been a focus at this year's Group of Seven (G7) summit in Hiroshima.
Some of China's lending has left developing countries "trapped in debt", U.S. Treasury Secretary Janet Yellen has said.
China, which has lent hundreds of billions of dollars to build infrastructure in developing countries, has called those remarks "irresponsible" and said the United States should take practical action to help developing countries.
Japan and Germany have for years been pushing for a reform of the Security Council.
The initiative comes as Japan and other G7 members try to engage the Global South, as the G7 nations struggle to stay relevant amid growing Chinese influence in developing countries.