HONG KONG — Rex W. Tillerson’s call for China to be denied access to its artificial islands in the South China Sea, made Wednesday during his confirmation hearing for secretary of state, set the stage for a possible crisis between the world’s two biggest economies should his comments become official American policy.

Mr. Tillerson told members of the Senate Foreign Relations Committee on Wednesday that China’s multibillion-dollar island-building campaign in the oil-and-gas rich sea was illegal and “akin to Russia’s taking of Crimea.”

“We’re going to have to send China a clear signal that, first, the island-building stops,” Mr. Tillerson told the senators. “And second, your access to those islands also is not going to be allowed.”

Should those words be translated into action after Donald J. Trump assumes the presidency on Jan. 20, it would be a remarkable change in the American approach to Beijing’s island-building in the South China Sea, which is transforming the area into what one Washington think tank said would by 2030 become “virtually a Chinese lake.” China asserts sovereignty over most of the South China Sea despite competing claims by countries including Vietnam and the Philippines and an international ruling rejecting most of Beijing’s assertions.