The accident took place at 9.49am in Anseong (local time), about 70km from Seoul, when 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.
The National Fire Agency said that three people had died and six were injured, including five who were in a critical condition.
Authorities were also searching for another person who was thought to be buried under rubble at the site, it said.
Broadcaster YTN aired dramatic footage showing a deck of a towering bridge collapsing at the site.
Hunting for further survivors, rescue workers at the site were seen examining twisted metal struts and fractured concrete slabs below the columns of the highway bridge.
Acting President Choi Sang-mok called to mobilise all available personnel and resources to rescue any missing people and to ensure safety measures to prevent further damage, his office said.
The Transport Ministry said it has dispatched a team of officials to the scene.
