The 7:40 a.m. blast at the facility in Phu Tho province caused tremors as far as 4 1/2 miles away, Thanh Nien Daily reported.
Fireworks were reported exploding even as rescue efforts were under way at the site, known as Z121, operated by the Ministry of Defense.
A police officer said about 2,000 people living near the plant had been evacuated, Sky News reported.
Phi Xuan Trung, chairman of the local commune, said "there were continuous explosions for some hours."
About 300 workers are at the plant during the week, said Maj. Gen. Le Quang Dai, head of the provincial military headquarters.
The cause of the blast has not yet been determined.
The plant is the only facility in the country that produces fireworks.