At least 14 people have been killed and eight injured in an attack on a drug rehabilitation centre in the northern Mexican city of Chihuahua.

Masked men broke into the Familias Unidas centre at 21:00 local time (02:00GMT) and opened fire on those inside.

Witnesses said the attackers wore military-style clothes and carried assault rifles.

Police say the motive for the attack is not yet clear.

The security forces erected checkpoints around the city but have so far not been able to track down those responsible.

Drug rehabilitation centres have been targeted by gunmen on a number of occasions.

In 2012, 11 people were killed in a drug rehabilitation centre in the northern state of Coahuila.

Two years previously, 13 people were shot in similar circumstances in the border city of Tijuana.

Investigators say drug gangs often target the clinics when they suspect rival gang members of hiding there.

They also suspect that the gangs use the clinics to recruit hitmen and smugglers, often threatening to kill those who fail to co-operate.

The state of Chihuahua, which borders with the United States. has seen a rise in murders in the past months which police say is down to a fight for control of drug markets between the Juárez and the Sinaloa cartels.