A fire at a prison in south-western Colombia has killed at least 49 people and injured a dozen more, prison authorities reported.

The director of the national prison system, Tito Castellanos, told Radio Caracol that it is not clear if all of the dead were prisoners.

He said the fire broke out during an attempted riot early on Monday at the medium security prison in the city of Tulua.

He said inmates had set mattresses on fire without considering the consequences.

President Ivan Duque expressed condolences to the families of those who died and said he had ordered investigations into the cause.