QINGDAO, China—The commander of U.S. naval forces in the Pacific said China had taken several destabilizing steps in the South China Sea—including conducting air patrols and announcing joint drills there with Russia—since a tribunal ruling against Beijing’s maritime claims in July.

Adm. Scott Swift, commander of the Pacific Fleet, also cited recent activities by Chinese fishing vessels in the East China Sea and reports that Beijing had erected hangars for military aircraft on artificial islands it has built in the South China...