The number of murders in Mexico linked to organised crime has jumped by almost 50% so far this year to 1,378, according to Mexico's attorney general.

Eduardo Medina Mora also said more than 4,000 people have died since President Felipe Calderon took office 18 months ago, declaring war on the drug cartels.

About 450 of those were police, soldiers, or prosecutors.

It comes as police in northern Mexico found four severed heads in ice-chests outside a motorway convenience store.

Five bodies - some decapitated - were also discovered in a city on the border with the US state of Texas.

Police attend the scene of a shoot-out between two drug gangs in northern Mexico.

The attorney general said many of the recent killings have been concentrated along the US border, while there have been fewer homicides in central Mexico.

The government says the violence is a symptom of the drug gangs' desperation amid the nation-wide crackdown involving more than 20,000 soldiers and police.

"Evidently when they are cornered and weakened, they have to respond with violence," Mr Medina Mora said in an interview on local radio.

In recent weeks, at least six senior police chiefs have been murdered - the most prominent being the assassination of the acting head of Mexico's Federal Police Force.  

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Mexican soldiers

BBC