Fifteen people were feared dead after a ventilation grate collapsed Friday during a concert by a popular girls' band near South Korea's capital, officials said.

Fire officials said two people had been confirmed dead and 13 others were unresponsive after the accident in Seongnam, just south of Seoul. They said 10 others were seriously injured.

Photos of the scene showed a deep concrete shaft under the broken ventilation grate.

The officials said the victims were standing on the grate while watching an outdoor performance by girls' band 4Minute, which is popular across Asia.

The officials spoke on condition of anonymity, citing department rules.

About 700 people had gathered to watch the concert, which was part of a local festival, the Yonhap news agency reported.

The YTN television network, citing unidentified witnesses, said many of the spectators were female students. The victims weren't immediately identified.

The cause of the accident wasn't immediately known.

The accident came as South Korea is still struggling to deal with a ferry disaster in April that left more than 300 people dead or missing.