Taipei has previously expressed dissatisfaction with China's modifications to the M503 route, asserting that any changes should be mutually agreed upon.

In response to the extension, Taipei condemned China's actions, emphasizing that alterations to flight routes should require mutual agreement.

On July 6, 2025, China announced the opening of a third extension, the W121, to the M503 flight route near Taiwan, which Taipei views as a unilateral attempt to alter the status quo in the Taiwan Strait.

The announcement of the W121 extension comes just days before Taiwan's annual Han Kuang military exercises, scheduled to begin on July 9, 2025, which aim to simulate responses to a potential Chinese invasion.

Historically, the median line in the Taiwan Strait has served as an unofficial boundary between Taiwan and China, but China does not recognize it and has increasingly disregarded it with military flights.

The W121 extension follows the previous openings of W122 and W123 in 2024, which were also positioned south of W121, all extending from mainland China towards Taiwan.

China's Civil Aviation Authority claims that the new route will optimize airspace efficiency and improve flight safety, asserting benefits for both sides of the strait.

Taiwan's Mainland Affairs Council criticized China's justification for the extension, arguing that it increases regional unrest and is particularly unjustified given that international air travel has not yet returned to pre-pandemic levels.

The M503 route has been a point of contention for Taiwan, which accuses Beijing of leveraging civil aviation routes for political and military purposes.

Taiwan's Mainland Affairs Council has not yet commented on the situation, reflecting ongoing tensions in cross-strait relations.

Despite China's claims of improving operational efficiency, Taiwan's officials argue that these unilateral actions exacerbate regional tensions.

Taiwan disputes China's assertions regarding the benefits of the flight route extensions, highlighting the current state of international air travel from mainland China.