Five people have died and two others were injured on Monday after an explosion and fire at a defense company in South Korea, official said.

The incident took place on production line at a factory for rocket propellant operated by Hanwha Aerospace 012450.KS, located in the city of Daejeon. The cause of the explosion is not currently known.

Emergency official Yoon Seong-su said that the area is a government-designated security facility. At least one of the people injured is in critical conditions, medical official Kim Ju-yeon said. It is understood the victims have not been identified yet.

The five dead included two temporary workers in their 20s and all were employed by Hanwha, the company said. The company’s chief executive officer Son Jae-il said “we bow our heads in apology to the victims and their bereaved families”.

Seoul immediately stopped operations at the security facility and deployed a massive Level 1 emergency response to the incident.

The site is one of Hanwha Aerospace’s key facilities where it develops large-sized propellants and surface-to-surface weapons systems, according to Yonhap news agency.