A powerful blast has rocked a US-owned sweets factory in northern Mexico, killing at least one person and injuring dozens more, officials said.

About 300 people were working at the Dulces Blueberry factory when the blast happened on Thursday in Ciudad Juarez, a city bordering the US state of Texas, according to official reports.

A city government spokesman initially said a boiler had exploded, but civil protection chief Fernando Motta Allen told the AFP news agency later that the cause and source of the blast were under investigation.

The blast took place on the second floor of the factory and caused the floor to collapse, injuring people working downstairs, said factory worker Ismael Bouchet.

"I was able to help five people who walked out of the building but as soon as they were out they went into shock and fainted," he said outside the factory, which produces gummy bears, jelly beans, peach rings and other sweets.

Al Jazeera's Adam Raney, reporting from Mexico City, said that local reports placed the number of injured at 51, with 11 in critical condition exhibiting second and third-degree burns.

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

Worried relatives of workers stood behind a security cordon as rescuers searched for people believed to still be inside the building.