The Zetas cartel has become the largest drug gang in Mexico, overtaking its bitter rival, the Sinaloa cartel.

The report by US security firm Stratfor says the Zetas now operate in more than half of all Mexican states.

Stratfor says the Zetas' brutal violence seems to have given the gang an advantage over the Sinaloa cartel, which prefers to bribe people. Since 2007, 47,500 people have died in drug-related violence in Mexico. The report says that drug-related violence in Mexico has persisted, despite the government's efforts to fight the cartels.

The report's authors say the violence has shifted, abating in some cities while worsening in others.

It lists the cities of Veracruz, Monterrey, Matamoros and Durango as examples of places where violence has increased, while murders in Ciudad Juarez have dropped, although the city remains the most violent in Mexico.

According to the study, most smaller drug gangs have joined either the Zetas or the Sinaloa cartel, turning the two groups into the predominant criminal forces in Mexico.

The Zetas control much of eastern Mexico, while the Sinaloa cartel has its stronghold in the west of the country.

The authors also point out their differences in strategy. They say that the Zetas resort to extreme violence, while the Sinaloa cartel, although also ruthless, prefers to bribe and corrupt people.

The report forecasts a continued expansion of Mexico's cartels into South America, a strategy which "eliminates middlemen and brings in more profit".

Its authors do not believe the Mexican government can eliminate the cartels "any more than it can end the drug trade".