An Arkansas nuclear plant was offline Monday after an explosion and fire in a transformer at the site, but there were no safety threats, operator Entergy said.

A witness was cited by the local River Valley Leader newspaper as reporting a "loud, ground-shaking explosion" and then smoke Monday morning at the Arkansas Nuclear One (ANO) plant in Russellville in the southern state.

Entergy said the 33-year-old, 987-megawatt number two unit at the plant was taken offline by the fire, but the 836-megawatt number one unit remained online.

"The fire has been contained, with no injuries or threats to safety. ANO Unit 1 remains online at this time," a spokesperson for the plant told AFP.