The disaster happened as over 5,000 students from six other schools were taking their final exams at the examination centre located in Barthelemy Boganda high school in Bangui, the Education Ministry said.

A hospital source told Reuters that 29 people were killed and 260 injured.

"We were really overwhelmed. It was terrible. Dozens of victims were arriving," the source said, recalling the stress his team felt.

Another medical source, who put the death toll at 31, said the sound of the explosion made students think the building was going to collapse.

"There was panic and some of the students jumped from the first floor," he told Reuters, adding that the test takers were roughly 18-22 years old.

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The Education Ministry said in a statement that a transformer located in the main building of the school exploded after a team from the country s main utility ENERCA restored electricity.

The statement also said the incident resulted in "a number of serious injuries, unfortunately including some fatalities".

The ministry said it would look into how the incident took place and promised to ensure a smooth continuation of the examination session.