Law enforcement authorities report..at least 15 people were killed and 18 others seriously injured Sunday when a trailer slammed into an express bus on the Seoul-Busan expressway in Cheonan, South Chungcheong Province.

The accident occurred at 2:40 p.m., when the 18-ton trailer that was heading towards Seoul crossed the median strip and collided with the bus.

The bus driver was among the 15 who perished. The death toll might increase as many injured people, who were whisked to nearby hospitals in Cheonan, suffered serious injuries. The 18 included seven very serious and 11 others with lesser injuries.

The accident caused expressway traffic in both directions to be blocked until 4:20 p.m. and remain extremely congested afterwards.

Police officials are looking into further reasons for the accident.