Thirteen die in Mexico prison fight in San Luis Potosi

At least 13 people are dead after a battle broke out between prisoners at a jail in the central Mexican state of San Luis Potosi.

A group of inmates used homemade knives and picks to attack rivals at the La Pila prison, the state attorney general's office was quoted as saying.

Authorities took several hours to bring the fighting under control.

Deadly outbreaks of violence are common in Mexico's overcrowded jails, which house inmates from rival drug gangs.

Human rights groups say the penal system suffers from chronic overcrowding and is in urgent need of an overhaul.

Despite assertions that the prison system would be reformed after the last major incident, in which 44 inmates were killed, there have been no tangible improvements, neither during the final year of the previous administration nor in the first six months of President Enrique Pena Nieto's government, says the BBC's Will Grant in Mexico City.