Police in Uganda say10 students died and 30 students were injured when a boys dormitory at a boarding school was set ablaze after the doors were padlocked.

Three former students expelled for disciplinary reasons and a school guard have been arrested in connection with the Monday fire at St. Bernard Secondary School in Rakai district in Uganda's southwest, police said Tuesday.

Uganda Deputy Police spokesman Patrick Onyango told CNN the students who were arrested were part of a group of 15 expelled by school authorities this year. Some of them had threatened to harm students in the school after they were expelled, police said.

Police said the death toll might rise as many students who sustained burns are in critical condition at hospitals and might not survive.

Onyango said early investigations into the incident revealed that "the arsonists locked the dormitory doors with padlocks to ensure none of the occupants escaped the inferno."

"By the time rescuers, who live in the neighbourhood, responded by breaking the locks, several had died, while others suffocated due to smoke that filled the rooms," Onyango told CNN.

"So far, we have been able to hand over the 10 easily recognizable bodies to parents, but after conducting DNA tests," he added.

The Uganda Red Cross Society in a statement said rescue teams used a hoe to break the padlock and rescue those trapped in the fire. URCS said many bodies were burnt beyond recognition.

It added that burial might be delayed until DNA tests have been concluded to ensure that families bury the right bodies.