Firefighters extinguish the fire at the primary lithium battery manufacturing plant in Hwaseong, Gyeonggi Province, Monday. (Yonhap)

The 23 deaths in the battery plant fire this week were found to have been caused by asphyxiation, according to officials on Thursday.

The Gyeonggi Nambu Provincial Police's investigation team for the accident at local battery manufacturer Aricell, the operator of the lithium battery plant, said that they received a verbal autopsy opinion from the National Forensic Service that it believed the victims had died of asphyxiation due to the fire. Asphyxiation happens when a substance such as carbon monoxide stops a person's body from getting enough oxygen.

The preliminary finding comes after police requested autopsies on the 23 people who died to determine the exact cause of death, authorities explained.

Apart from investigating the causes of death, the forensic agency is also working on identifying the victims through DNA sampling and comparison, which is expected to take time, according to police.

Police added that they have identified three more people unaccounted for in the fire, bringing the total for identified fatalities to 17. The three deceased identified were all Chinese women.

The fire engulfed a primary lithium battery manufacturing plant in Hwaseong, Gyeonggi Province, Monday. It spread rapidly as the battery cells inside continued to explode, making it difficult for rescuers to go inside and search. At least 35,000 batteries were believed to be inside the plant.