The United States has questioned Japan about the possibility it might participate in China's initiative to set up a regional development bank, sources familiar with bilateral relations said Friday.

In the wake of a Financial Times report at the end of March that said Japan "is likely to join the Asian Infrastructure Investment Bank within a few months," in a move it said "would see Tokyo break ranks with Washington," the U.S. has grown suspicious about Japan and asked whether the report is true, the sources said.

The development shows that Washington's concern is growing since major allies including Britain, Australia and South Korea filed applications to become "founding members" of the AIIB before China's March 31 deadline for countries and territories.