An attack on a drug rehabilitation centre in northwestern Mexico has left at least 11 people dead and nine others wounded, according to police.

The attack on Sunday evening in the city of Torreon is believed to have been carried out by contract gunmen, officials said.

"The armed attackers came on two pickup trucks, entered the centre and opened fire," a senior police official, speaking on condition of anonymity, told the AFP news agency.

The gunmen used assault rifles and handguns, the police noted, saying more people could have been wounded in the attack.

According to witnesses, some of those wounded walked away from the centre and disappeared before the police arrived.

The dead have not been identified yet.

The assault took place nearly a year after an attack on a similar rehabilitation centre in Torreon, located in Coahuila state, that left 11 people dead and two wounded.

In 2010, five drug rehabilitation centres - all in the north of the country - were attacked.

More than 50,000 people are believed to have been killed in drug-related violence in the Central American country since 2006.

That year the government launched a military crackdown on the country's powerful drug cartels, which are themselves locked in brutal turf wars marked by violence such as beheadings and mass graves.

The government's official count of more than 47,000 dead has not been updated since last September.

Source: Agencies