At least 16 people lost their lives today and 5 are still missing following a fire that broke out in lithium battery manufacturing plant in South Korea, according to an account released by the fire service.

The fire, which has been mostly extinguished, broke out around 10:30 local time (04:30 Greek time) at a factory of battery maker Aricell in Hwaseong, south of the capital Seoul.

The fire broke out after battery pack explosion inside a warehouse that housed 35,000 units, fire department official Kim Jin-young said. It is currently unknown what triggered their explosion, he added.

An eyewitness told Reuters he saw firefighters carrying up to six bodies out of the factory. Due to the intensity of the fire, rescuers are having difficulty identifying the dead, Kim added.

South Korea’s Yonhap news agency had earlier reported that about 20 bodies had been found inside the factory, but Kim noted during a televised briefing that 16 people had died and 2 had burns and other serious injuries.

The fire department spokesman also noted that rescuers are inside the factory and are making efforts to find the five people who are missing.

Kim Jae-ho, a professor of fire and disaster prevention at Taejeon University, said the fire likely spread too quickly, preventing workers from escaping.

“Battery materials, such as nickel, are highly flammable,” he said. “Very often there is not enough reaction time, compared to a fire caused by other materials,” he explained.

South Korean President Yun Suk-yeol is monitoring the situation, according to a statement from his office, while Interior Minister Lee Sang-min has asked local authorities to take measures to prevent contamination of the area around the plant by any dangerous chemicals.