A large fire on a South Korean expressway on Thursday killed at least five people and injured nearly 40 caught in heavy traffic, emergency officials and media said.

Video images on social media showed a soundproof awning engulfed in flames soon after the fire broke out on the Second Gyeongin Expressway near the capital Seoul at about 1:49 pm (0449 GMT).

The heavy traffic made it difficult for vehicles to escape the affected area as the fire spread and a blast was heard, a witness told television broadcaster YTN.

Media said the fire appeared to have begun after a bus and a truck collided. But a fire official at the scene said its cause was still being investigated, although the fire had been put out. Altogether 44 vehicles were destroyed in the fire, he said.

Five people were killed with at least 37 injured, three of them seriously, an emergency official said.