TAIPEI (Taiwan News) — Two Shenyang J-11 fighter jets and a Harbin Z-9 anti-submarine helicopter entered Taiwan’s air defense identification zone (ADIZ) just hours after the Chinese aircraft carrier Shandong sailed through the Taiwan Strait, the Ministry of National Defense said Friday (March 18).

The aircraft were spotted during the day in the southwest sector of the ADIZ off the coast of Taiwan’s main island, the Liberty Times reported. Because there was no land close to the area where the helicopter appeared, speculation was that it had taken off from a Chinese navy vessel.

Taiwan’s Air Force jets issued radio warnings telling the Chinese aircraft to turn back immediately. The military also said it deployed air defense missile systems to monitor the activities of the People’s Liberation Army Air Force jets and helicopter.

China’s Shandong carrier earlier sailed past the island of Kinmen before heading north through the Taiwan Strait, Reuters reported. The United States guided missile destroyer USS Ralph Johnson was shadowing the Chinese ship, while vessels from Taiwan’s Navy were also present in the area, according to the report.