Long before an unmanned Chinese airship floating over the US grabbed the world’s attention, Taiwan may have glimpsed Beijing’s ambitions to turn balloons – seemingly so old-fashioned and ponderous – into elusive tools of 21st century military power.

Residents in Taipei and elsewhere on the island have spotted and photographed mysterious pale orbs high in the sky at least several times in the previous two years. But few people here, even officials, gave them much thought then. Now, Taiwanese officials are grappling with whether any of the balloons were part of China’s growing fleet of airborne surveillance craft, deployed to gather information from the self-ruled island that Beijing claims as its own.