The incident occurred around 9.50am local time (8.50am, Singapore time) in Anseong, around 65km south of the South Korean capital.
Five 50m steel structures supporting the highway bridge collapsed one after another after being hoisted into place by a crane, the Yonhap news agency reported.
Rescue workers at the site were seen examining twisted metal struts and fractured concrete slabs below the columns of the highway bridge.
All individuals who suffered injuries are currently being treated at hospitals,” the National Fire Agency’s spokesperson said.
The National Fire Agency had initially said it took place in the nearby city of Cheonan but later corrected that claim.
Hyundai Engineering, the main contractor of the construction site, apologised and said it was assisting authorities.
“We sincerely bow our heads in apology to those who lost their precious lives and were injured in the accident at our construction site,” it said.
“We are actively cooperating with the relevant authorities to ensure a prompt resolution on-site” and to identify the cause of the accident, the company said.
Acting President Choi Sang-mok has issued urgent directives to mobilise all available resources and personnel for the rescue operation.
The National Fire Agency said it mobilised three helicopters and nearly 150 officials for the search and rescue operation and the Transport Ministry said it dispatched a team of officials to the scene.
