CIUDAD JUAREZ, Mexico (AFP) - A huge blast rocked a United States (US)-owned candy factory in northern Mexico on Thursday, causing the ceiling to collapse, injuring more than 40 people and leaving 20 more missing, officials said.

Around 300 people were working at the Blueberry factory when a boiler exploded in Ciudad Juarez, a city bordering the US state of Texas, officials said.

The second floor ceiling crashed and the explosion completely destroyed a side wall of the building.

"We are searching for people in the debris. We calculate 20 have disappeared," city government spokesman Guillermo Terrazas told Agence France-Presse.

Civil protection chief Fernando Motta Allen said 10 of the injured people were in serious condition and another four were in a critical state.

"It is possible that some people were trapped after the ceiling collapsed due to the explosion," he said.

"There is an area where (rescuers) were not able to get to because they are working to control the fire sparked by the damaged boiler."

Some 30 ambulances rushed to the scene of the explosion to take the injured to hospitals.