A popular 140-metre-long tourist bridge has collapsed into a harbour in the north-east town of Yilan in Taiwan, crushing several fishing boats, and authorities fear there could be a number of casualties.

Key points: A 140-metre-long bridge collapsed onto fishing boats below

A 140-metre-long bridge collapsed onto fishing boats below 10 people were taken to hospital, six with serious injuries

10 people were taken to hospital, six with serious injuries Five people were thought to have been on the bridge when it collapsed, with six believed trapped on one of the boats

"[Nanfangao bridge] collapsed around 9:30am while an oil tanker vehicle was on it, setting the vehicle on fire," Town Mayor Shih I-chun said.

The weather at the time was sunny, but the collapse occurred hours after a typhoon swept by the island.

Disaster relief officials would not say if the storm had weakened the bridge or give other details on the potential cause.

The centre of Typhoon Mitag passed east of Taiwan on Monday night and early Tuesday (local time), but the outer winds gusting up to 137 kilometres per hour affected the island before the storm moved to the north-east.

Fishing vessels were sent to help search for the missing. ( AP )

The storm injured 12 people, felled trees, damaged vehicles and caused power outages around the island.

At least 10 people have been taken to hospital, six with serious injuries, following the bridge collapse on Tuesday.

Divers plunged into the waters to search for those feared trapped in at least two fishing boats, two government officials from the port township of Suao told Reuters.

At least nine people fell in the water and seven were rescued, Taiwan's Central News Agency said.

Space to play or pause, M to mute, left and right arrows to seek, up and down arrows for volume. Watch Duration: 22 seconds 22 s Bridge collapse in Taiwan leaves dozens injured

An air force helicopter, fishing vessels and more than 60 military personnel including marines and naval officers were also deployed in the search.

"Saving life is priority," Taiwan Premier Su Tseng-chang said in a Facebook post.

"I will ask related authorities to make all effort for the rescue."

Rescuers help one of the people injured in the bridge collapse. ( AP )

Up to six people are believed trapped on one of the boats, and five people were feared to have been on the bridge when it collapsed.

National Fire Agency spokesperson Su Hong-wei said the tanker's fall smashed three boats, injuring six Filipino nationals and three Indonesians, with the nationality of the last victim unknown. People from both countries regularly staff Taiwanese-registered boats.

Officials have not yet determined what caused the bridge to collapse. ( AP )

It also set off a fire on the tanker truck, but did not spread beyond the vehicle.

Taiwan President Tsai Ing-wen said she hoped all Government departments would do everything possible to save people and "keep the number of deaths and injuries as low as possible," CNN reported.

More than 60 military personnel have been deployed in the search. ( AP: Taiwan's Coast Guard )

According to the company that designed the 18-metre-high bridge, MAA Consultants, it is the only single-span arch bridge in Taiwan supported by cables and the second single arch-cable steel bridge in the world.

Nanfangao Bridge is a tourist attraction in Yilan. It was opened in 1998 and was built to replace a lower bridge that prevented large fishing vessels from passing underneath.

Reuters/AP