The Taiwanese Civil Aeronautics Administration (CAA) has warned Chinese carriers not to use four air routes unilaterally opened by China over the Taiwan Strait as operations there can pose a safety danger, The Taipei Times has reported.

The CAA said that by allowing northbound traffic to use the M503 route, as well as by unilaterally opening the three extension routes W121, W122, and W123 to air traffic, China has violated the terms of a 2015 agreement between Beijing and Taipei.

While all four routes are within Chinese airspace and have been certified by the International Civil Aviation Organisation (ICAO), the M503 route lies only 7.8 kilometres from the cross-strait median line, a disputed line of control between the People's Republic of China and the Republic of China (Taiwan). Under the terms of the 2015 agreement, both governments agreed to use the route for southbound traffic only and not to activate the extension routes without prior agreement.

The corridors are used by the Civil Aviation Administration of China (CAAC) Air Traffic Management Bureau to alleviate congestion on the A470 route, used by flights connecting the Yangtze River Delta, where Shanghai Pudong, Shanghai Hongqiao, Nanjing, and Hangzhou are located, among others, with the Pearl River Delta area, encompassing Hong Kong Int'l, Shenzhen, and Guangzhou and other airports.

The Taiwanese CAA believes that carriers operating northbound flights on the M503 route could disrupt military traffic along the median line, while the extension routes could conflict with cross-strait operations. It has appealed to the Chinese authorities to resume negotiations regarding the use of these routes.

According to the CAA's data, the routes are mostly used by Cathay Dragon, Hong Kong Airlines, China Eastern Airlines, and Xiamen Airlines.