The blast, which is believed to have been caused by a pressurized reaction kettle used in manufacturing, occurred during a changeover in shifts.

A statement released by the emergency response office of Jiangxia district said the injured workers were taken to three hospitals for treatment. Two were in serious condition.

A firefighter surnamed Li said rescue services were busy throughout the afternoon searching for survivors among the debris.

A man surnamed Hu who lives near the factory said the windows of his seventh-floor apartment were broken by the power of the explosion.

The Lucheng factory, which produces a coloring agent used in the food and pharmaceutical industries, employs 56 workers.

A reaction kettle is a container in which raw materials are heated, distilled and cooled.

The cause of the explosion is still being investigated.