By Agencies

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Violence at a prison on the outskirts of Mexico's northern business city of Monterrey has killed at least 20 people, authorities said.

A fight broke out in the early hours of Sunday morning and prisoners started a fire by setting mattresses alight.

Pablo Gomez, a spokesperson for the state of Nuevo Leon, said it was unclear what caused the riot.

Gomez said that the violence was brought under control.

The prison houses some 3,000 inmates, said Jorge Domene, a senior Neuvo Leon state official.

Powerful drug cartels are fighting for control of smuggling routes along the US-Mexico border and rivalries often spread into the prison system, where prisoners held on federal drug charges are mixed with common criminals.

A fight between rival gangs in another prison in the north of the country left 31 inmates dead in January and a further 13 wounded.

About 50,000 people have died in Mexico in the past five years since President Felipe Calderon launched a frontal attack on drug gangs. Packed and inefficient prisons are plagued with corruption and mass escapes.