Eleven out of 12 workers buried under the debris of a collapsed gymnasium construction site were rescued while the search for the one missing worker was still under way, firefighters said Wednesday.

The accident took place at around 5:05 p.m. in the southern district of Sadang, where the construction for a new public gymnasium had been in progress.

Workers were pouring concrete into the mold of ceiling slabs when a few of them fell along with part of the ceiling that suddenly caved in and they were buried with others who were working underneath it at that time.

They were trapped for a little over an hour before they were rescued by the firefighters and sent to nearby hospitals.

None has sustained any serious injuries from the collapse, with a few of them appearing to have suffered leg injuries, firefighters said.

Firefighters earlier had said there were nine people buried under the debris, but they found three more victims during the rescue.

The new Sadang gymnasium had been under construction since June 2013, headed by the district office, and was scheduled for completion by the second half of this year. (Yonhap)