BEIJING—China has sent an uninvited surveillance ship to international waters off Hawaii to monitor U.S.-led naval exercises, even though the Chinese navy is participating in the biennial drills, the U.S. Pacific Fleet said Saturday.

China's debut at the monthlong Rim of the Pacific, or Rimpac, exercises has been hailed by Chinese and U.S. officials as evidence of an improving military relationship, despite escalating tensions over territorial disputes in Asia.

...