Three firefighters were killed while battling a warehouse blaze in Pyeongtaek on Thursday as authorities were still struggling to bring the fire under control after over 12 hours, officials said.

The bodies of two deceased firefighters were found at 12:22 p.m. on the second floor of the refrigeration warehouse construction site in western Pyeongtaek, a city located 70 kilometers south of Seoul. The third firefighter was discovered nearby about 20 minutes later.

The fire at the seven-story warehouse was first reported at 11:46 p.m. Wednesday. Authorities reportedly brought the blaze under control Thursday morning but the fire suddenly began to rise again later.

The firefighters were believed to have been trapped due to the fast spread of the fire. Two other firefighters managed to escape from the hazard and were moved to a hospital after having inhaled smoke.

Authorities have issued a level 2 warning, which enables them to call in support, including firefighters, from other areas. Five civilian workers at the scene have reportedly been evacuated.

"The firefighters were presumably trapped due to the sudden combustion and internal destruction within the site," an official said.

President Moon Jae-in expressed condolences over their deaths, saying he is "heartbroken" by the news of three firefighters "who died while engaging in rescue activities at the forefront of protecting the lives and safety of people," according to presidential spokesperson Park Kyung-mee.

Lee Jae-myung, the presidential candidate of the ruling Democratic Party, and his rival Yoon Suk-yeol of the main opposition People Power Party, expressed condolences and called for measures to ensure firefighters' safety.

"I have said firefighters work on behalf of God because they are always at the forefront of the fight against dangerous disasters to protect people's lives and safety," Lee said, adding that the country should work to keep them safe.

Yoon said the next government must come up with measures to protect firefighters who "devote themselves day and night for people's lives and safety." (Yonhap)