The explosion at Kilgore Flares Co. was reported just before noon, said Cris Hill, a dispatcher at the Hardeman County sheriff's office.
A Memphis hospital reported that three people were brought there in critical condition and a smaller hospital in Bolivar reported that three people there were in good condition.
County Mayor Willie Spencer, who was at the scene, said the fire apparently was contained to one building, which was heavily damaged.
"It could have been a lot worse," he said. "Any time you have an explosion ... you never know how many people are around."
As for the cause, Spencer said, "I don't think they have a clue."
An operator who answered the phone at the plant's facility about 75 miles northeast of Memphis said the company could not release any information.
The company website says Kilgore supplies infrared decoy flares to counter the threat of guided missiles. The company announced earlier this year a $22.5 million Department of Defense order for flares for B-52 aircraft.
A worker at the Kilgore plant was killed during a flash fire and explosion in April 2001.
The company was bought in 2001 by Chemring Group PLC, a British defense company.