The Nanfangao Bridge – a tourist attraction in Suao, Yilan – fell on Tuesday morning while an oil truck was crossing it.
The vehicle and the road crossing on the 60ft-high bridge fell and crushed several boats in the fishing harbour below.
Footage posted on social media showed a large truck almost getting across the bridge before it tumbled backwards as the bridge collapsed into the water.
More than 60 army and naval personnel, including divers, are involved in the search for potential victims and survivors.
More than 60 army and naval personnel were involved in the rescue operation
Six people were feared trapped on one of the vessels, while five are believed to have been on the bridge at the time it collapsed.
A further 10 people were taken to hospital for treatment – six of them with serious injuries.
An investigation has been launched into the cause of the accident, which happened hours after a typhoon swept across parts of the island.
It was understood one of the bridge’s piers may have collapsed, the state-run Central News Agency reported.
President Tsai Ing-wen said she hoped all government departments would do everything possible to “keep the number of deaths and injuries as low as possible”.
The 460ft long bridge, which opened in 1998, was built to replace a lower bridge that prevented large fishing vessels from passing underneath.
It is the only single-span arch bridge in Taiwan supported by cables and the second single arch-cable steel bridge in the world, according to its designers, MAA Consultants.
