A Taiwanese fighter jet crashed and its pilot was killed Thursday while training over the Western Pacific, highlighting concerns about the island’s aging military fleet at a time of increased pressure from China’s air force.

Rescuers are searching for a Taiwanese F-5 jet after it plunged into the sea just a few hundred meters off the east coast of Taitung county during a training exercise Thursday morning, according to a Defense Ministry spokesman. The pilot, who had ejected and was rescued, later died despite hours of emergency treatment.

The incident fuels mounting worries about the reliability of the older aircraft in Taiwan’s aging military fleet at a time of near-daily Chinese incursions into the island’s air defense identification zone.