The fire allegedly broke out when a truck and a bus collided in an automobile tunnel south of Seoul, South Korea, South Korean news agency Yonhap wrote.
The flames spread rapidly inside the tunnel, causing a huge smoke.
At least six people were found dead on the tunnel bridge. 20 people are being treated after inhaling toxic smoke from the fire.
About 50 emergency vehicles and over 140 firefighters are working to put out the blaze. A rescue helicopter is also used in the work.
South Korean authorities warn that the death toll could rise further.
