A blast that levelled an explosives plant in rural Tennessee has left 19 people missing and feared dead.

Humphreys County Sheriff Chris Davis said the blast at Accurate Energetic Systems, which supplies the military, was one of the worst scenes he's ever seen.

He did not specify how many people were killed but referred to the 19 missing as "souls" and said officials were still speaking to family.

"There's nothing to describe. It's gone," Davis said. "It's the most devastating scene that I've seen in my career."

People reported hearing and feeling the explosion from kilometres away. The company's website says it makes and tests explosives at an eight-building facility that sprawls across wooded hills in the Bucksnort area, about 97km southwest of Nashville.

Davis said investigators are

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