“We have no choice but to be very cautious about participation,” Mr. Aso said.

Despite American objections, dozens of nations signed up ahead of the March 31 deadline set by China to become founding members of the new bank, to be based in Beijing. The most recent countries to join included Australia and South Korea, two of the United States’ closest military partners in the region. South Korea’s decision reversed earlier expressions of agreement to American requests to avoid the bank.

The decisions by so many allies to break with Washington have been seen as a sign of the growing financial clout of China, which has become the largest trading partner of many countries in the region. The new bank will confirm its founding members in April and aims to begin operating by the end of this year.

While there have been calls in Washington for the United States to join, the Obama administration is thought unlikely to reverse its opposition. Many American officials appear concerned that the new lender will undermine the World Bank and also the International Monetary Fund, two pillars of the global financial order established by the United States after World War II.

Although China has become Japan’s top trading partner, Tokyo has compelling security-related reasons for siding with the United States. Foremost among these is an intensified confrontation with China over control of uninhabited islands in the East China Sea that are claimed by both nations.

Eager to ensure that his nation has American backing in the standoff, Mr. Abe has tried to bind Japan more closely than ever to the United States, its postwar military protector. He is expected to emphasize Japan’s close friendship, and also its growing willingness to assume a larger military role in the region, when he visits the United States in late April.