A group of hired killers entered the bar Sabino Gordo, located in downtown Monterrey, killing at least 20 [EPA]

At least 20 people were killed when armed men sprayed gunfire on a bar in the northern city of Monterrey.

Alejandro Poire, a federal security spokesman, said on Saturday that the attack took place late on Friday.

Monterrey, a major industrial hub, has seen a spike of violence since the Gulf and Zeta cartels began fighting for control of drug trafficking there two years ago.

Poire said another 11 bodies were discovered on Friday on the outskirts of Mexico City and that 10 people were found dead early on Saturday in the northern city of Torreon.

He repeated the government's insistence that criminals - not the crackdown on organised crime - are causing the violence.

"The federal government strongly condemns the various homicides which took place in the cities of Valle de Chalco, Monterrey and Torreon in recent hours," he said.

More than 35,000 people have died since President Felipe Calderon stepped up action against organised crime in 2006, according to official figures. Some groups put the number at more than 40,000.

Source: Agencies