BEIJING — In a sign of the increased tensions between the United States and China on the open seas, navy vessels from the two countries almost collided in the South China Sea when a Chinese ship cut across the bow of an American cruiser, a senior United States defense official said on Saturday.

An accident was averted when the missile-carrying cruiser Cowpens, traveling in international waters, maneuvered to avoid the Chinese vessel, the official said. At the time, the American ship was observing China’s new aircraft carrier, which was also in the vicinity.

The near collision, which occurred on Dec. 5 but did not become public until Friday, was one more example of the growing rivalry between China, a rising maritime power, and the United States, the dominant naval power in the Pacific region since World War II.

The episode at sea came as the Obama administration has chastised China for imposing an air defense identification zone in the East China Sea over islands and airspace that are also claimed by Japan. In announcing the zone, the Chinese said they would require planes entering the area to file advance flight plans, a demand the United States and Japan have both defied.