A freight truck collided with a bus on a highway near the South Korean capital Seoul on Thursday, causing a fire that killed at least five people and injured 37 others, officials said.

It wasn’t immediately known what caused the collision and fire, nor what cargo the truck was carrying.

The National Fire Agency said in a statement that three of the injured were in serious conditions while 34 others were lightly injured. Fire officials said the collision occurred inside a noise-barrier tunnel on the highway.

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Earlier TV footage showed thick black plumes of smoke billowing from the tunnel, but fire officials said later they'd almost put out the blaze.

The public affairs office at the fire agency couldn’t immediately confirm where the five dead came from.