A surge in drug violence in northern Mexico has left at least 40 people dead in the last 24 hours.

In the Mexican city of Ciudad Juárez, masked men shot dead at least 17 patients at a drug rehabilitation clinic. Five were injured.

Ten gunmen forced their entry into the clinic near the US border, lined the patients up and shot them. Drug gangs from Ciudad Juárez often attack drug clinics because they believe rival gang members are hiding there.

Wednesday's attack is one of the bloodiest since Mexican President Felipe Calderón began a clampdown on drug cartels three years ago. Since then more than 13,000 people have been killed in drug violence.

In Morelia, the capital of the western state of Michoacán, a deputy police chief has been shot dead together with his bodyguards and a passer-by. It is likely that gang members carried out the attack.

During the night 21 people were killed in drug-related violence in the state of Chihuahua.

Photo Cuidad Juárez - Flickr/Charlois