Rescuers were scrambling on Tuesday to reach six people they feared were trapped after a bridge collapsed in Taiwan, smashing onto a group of fishing boats moored underneath.

Dramatic CCTV images captured the moment the 140-metre long single-arch bridge came crashing down in Nanfangao, on Taiwan's eastern coast.

In the images, the road gives way and tumbles down onto at least three fishing boats as a petrol tanker that was crossing also plunges into the water.

The National Fire Agency said they believed six people were unaccounted for and could be trapped in the boats underneath the collapsed structure.

"The rescue operation is ongoing," the agency said in a statement, adding at least twelve people were injured, including six Philippine and three Indonesian fishing workers as well as the Taiwanese driver of the petrol tanker.

"We hope to safely rescue all in the shortest time to minimise the damage," Taiwan's President Tsai Ing-wen told reporters.

The petrol tanker burst into flames at some point after it hit the water, sending a thick plume of black smoke into the air.

It is not clear what caused the bridge, which was built in 1998 and spanned a small fishing port, to collapse.

Taiwan was skirted by a typhoon on Monday night, which brought heavy rains and strong winds to parts of the east coast.

But at the time of the bridge collapse the weather was fine.

Taiwan has a huge fishing industry and many of those who work on its boats are low-paid migrant workers from countries like the Philippines, Indonesia and Vietnam.