Gunmen using assault rifles have shot their way through a drug rehabilitation center in northwestern Mexico, killing 11 people and wounding at least nine others , police said.

Dead bodies were strewn across blood-soaked beds, armchairs and floor at the church-operated rehabilitation center in the city of Torreon after the attack, Milenio television network reported.

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

The assault on the facility, known as "Your Life on the Rock", occurred at 9:15 pm Sunday (0215 GMT Monday), police officials said.

The gunmen used AR-15 assault rifles and nine-millimeter caliber pistols, the police added, saying more people could have been wounded in the attack.

According to eyewitnesses, some of the victims walked away from the center and disappeared before the police arrived.

The dead have not been identified yet.

The building, located in Torreon's residential area in front of a park, has been cordoned off by police, with forensic experts working the crime scene.

The army and police have also established security checkpoints around the hospital where the wounded were being treated in order to prevent contract killers from finishing off those who survived the attack and could serve as witnesses.

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

Five drug rehabilitation centers -- all in the north of the country -- were attacked in 2010.

These centers have become frequent targets of drug cartels, which seek members of rival trafficking groups among the patients and eliminate them to prevent them from talking to the police.

Mexico has been struggling with a massive crime wave in recent years, with more than 50,000 people believed to have been killed in drug-related violence 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 macabre displays of 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.

The Torreon region is a scene of a violent confrontation between the powerful Sinaloa cartel and Zetas, an upstart narco-trafficking group created by deserters from US-trained Mexican special forces.

The two gangs are fighting for control of lucrative cocaine delivery routes to the United States.

[Photo illustration. AFP Photo/Hector Guerrero]