Elevated parts of a highway under construction collapsed in South Korea on Tuesday, killing four workers and injuring six others, officials said.

Ten people were working on the site near the city of Cheonan, about 90 kilometres south of Seoul. They fell when it collapsed and were trapped in the rubble, the National Fire Agency said.

One was found dead in the rubble. Eight injured workers were sent to hospitals before two of them were declared dead there later. The 10th worker who was missing for hours was eventually discovered dead, according to fire agency officials.

The fire agency said in a statement that five remained in serious condition.

The cause of the collapse wasn't immediately known.

Acting President Choi Sang-mok urged authorities to mobilize all available personnel and equipment to salvage the workers. About 50 fire trucks, three helicopters and some 150 personnel were mobilized for the rescue effort.