The death toll has risen to six after a fire occurred in a sound-proof tunnel of the Gyeongin Expressway in Gwacheon, about 20 km south of the capital Seoul, Yonhap news agency said Thursday.
Three people were seriously injured while 26 others sustained minor wounds.
Images from Yonhap showed flames and a big plume of black smoke rising from the scene.
It's not immediately clear what caused the tunnel fire.