The accident took place at Pangyo Techno Valley, a multi-purpose complex for technology firms, in Seongnam, Gyeonggi Province, just southeast of Seoul. About 700 people were attending the concert organised by the provincial government.
"Twelve people were killed at the scene, two others were killed while they were being rushed to the hospital. Others are assumed to have passed away while receiving medical treatment," a fire official, who spoke on condition of anonymity, said on Friday.
Television reports said 25 people fell more than 10 metres into an underground parking area when the grate collapsed. The injured have been rushed to four hospitals near the venue. Most of the dead and injured were students, the YTN news channel said, adding that the concert organisers had repeatedly urged the spectators to move off the grate before it collapsed.
"There was a sudden, loud screaming, and when I turned it looked as if people were being sucked down into a hole," one witness told YTN.
A woman standing nearby said a great "cloud of dust" billowed up from the ventilation shaft after the grate collapsed. In February, the roof caved in on a student-packed auditorium near the southern city of Gyeongju, killing 10 people and injuring more than 100. An investigation uncovered evidence of structural flaws and lax management controls.
The deaths prompted government promises of a national review of safety standards, as it became clear that poor regulatory oversight was a major contributor to the scale of the tragedy.