BEIJING — China commissioned its first domestically built aircraft carrier on Tuesday, reaching a new milestone in its ambitions to build a modern navy capable of challenging American dominance of the seas, especially in the Pacific.

China’s leader, Xi Jinping, presided over a commissioning ceremony at the Chinese naval base in Sanya, on the southern island of Hainan, where the vessel will be based, according to video broadcast on China’s state television.

From there, the new carrier will be able to project power throughout the disputed islands of the South China Sea and the Taiwan Strait — a development that has been watched warily by the United States and other Pacific powers, including Japan and Australia.

“This is a major milestone for China,” said Matthew P. Funaiole, a senior fellow with the China Power Project at the Center For Strategic and International Studies in Washington who tracks the country’s military developments. “There are only a handful of countries capable of fielding aircraft carriers, and China now has two, which puts it in elite company.”