As the United States has increasingly backed away from multilateral leadership and engagement at the United Nations, China in particular has stepped up to fill the vacuum, according to diplomats and insiders, while countries such as Japan are left on uncertain footing.

"China has become much more eager to assert itself and look like a leader in the U.N. system," said Richard Gowan, a senior fellow at the United Nations University, said. "It's a strategic shift, and at a moment where the United States is pulling back, China has a lot of opportunities to make its power felt."

Part of China's rise in the international body is a direct result of its growing financial prosperity. In the calculation of each nation's funding obligations for the U.N. budget from 2019-2021 — an assessment that factors in gross national income, debt adjustments and other variables — China has come out ahead of Japan as the second-largest contributor, behind only the United States.