Four persons were killed and 10 others were injured on Wednesday when a subway construction site in South Korea collapsed.A rescue official told a briefing that the subway construction site in the city of Namyangju, east of capital Seoul, collapsed at about 7.27 a.m., Xinhua news agency reported.According to Yonhap News Agency, the accident was estimated to have been caused by an explosion of oxygen cylinders during welding works. However, the exact cause is still under investigation.Rescue operations are still going on for possible casualties.