A Hanwha Aerospace blast killed five people and injured two others on Monday at a rocket propellant production line in Daejeon, South Korean officials said.
Fire officials said an explosion triggered a blaze at the plant. Investigators had not yet confirmed the cause.
The two injured workers escaped from the facility, a fire official told a briefing. One survivor suffered serious burns.
Hanwha Aerospace said the blast appeared to occur while workers were using water to clean explosive material from tools used in rocket propellant production.
Chief Executive Officer Son Jae-il apologised to the victims and their families. He also pledged full cooperation with authorities.
The company said all five people who died worked for Hanwha. Two of them were temporary workers in their 20s.
Read: Deadliest Plane Crash in South Korea Claims 179 Lives at Muan Airport
South Korea, an East Asian country with Seoul as its capital, treats the Daejeon facility as a national security site. The plant produces large propulsion engines and handles rocket propellants.
South Korean President Lee Jae Myung ordered officials to mobilise all available resources. His office also called for an investigation into the accident.
Shares in Hanwha Aerospace fell 2.8% in afternoon trade. Hanwha Corp shares dropped 3.4% after the incident.
