No survivors are expected to be found after a major explosion at a Tennessee munitions factory on Friday that has left 18 people unaccounted for, BBC reported.

Recovery teams are still clinging to hope of finding any of the missing alive, but assume they are deceased, said Humphreys County Sheriff Chris Davis.

"As we get into this, we find it even more devastating than we thought initially," he told a news conference.

It's still unclear what caused the explosion at the plant in Bucksnort, Tennessee - roughly 56 miles (90km) south-west of Nashville. The facility specialises in the development and manufacture of explosives, according to BBC.