Funerals were held on Sunday for some of the victims of an attack on a birthday party in Ciudad Juarez [AFP]

Thirteen people have been shot dead at a drug rehabilitation centre near the Mexican border city of Tijuana, police say.

The motive for Sunday's attack was not immediately known, but it appeared to be linked to the drug war that has claimed thousands of lives this year.

"There were 13 people killed. They were all recuperating drug addicts," a police official said.

According to a witness, armed men lined up their victims against a wall and executed them in cold blood.

As many as 105 tonnes of marijuana were seized in Tijuana last week in the largest drug bust in Mexican history, and some law enforcement officials suggested the killings might have been connected to this event.

The rehabilitation centre is located in the same neighborhood where the drugs, worth more than $335 million on the Mexican street and even more if sold in the US, were seized and destroyed.

Teenagers killed

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Elsewhere in the country, a mother and her two teenage children were killed in Torreon in the northern Coahuila state, when men in vans opened fire on a group of police patrol cars. The officers and soldiers returned fire.

It was not clear who fired the shots that killed the bystanders, but the state attorney general's office said it was investigating and expressed condolences to the victims' families.

"Because they were driving where the shooting took place, a 14-year-old boy, his 18-year-old sister and their 47-year-old mother were killed," the office said.

None of the criminals or the police officers were wounded in the firefight and no arrests have been made, officials said.

Coahuila has been the scene of bloody turf battles between the Sinaloa cartel and the Zetas drug gang.

Party attack toll

Meanwhile, in the border city of Ciudad Juarez, the death toll from an attack on a birthday party rose to 13 after an 18-year-old male died of his injuries.

Memorial services were held for some of the victims and prosecutors said that guards had been provided to protect those services.

Prosecutors in Chihuahua state, where Ciudad Juarez is located, said they were searching for a man known only by his nickname, "The Mouse," who was apparently the target of the attackers.

The man was reportedly wounded in the Friday shooting, but has disappeared. Investigators said they believe he can shed light on who was trying to kill him.

The government of Felipe Calderon, the president, has come under fire from human rights groups lately for the growing civilian death toll the government's crackdown on crime and drug trafficking is causing.

More than 28,000 people have been killed across the country in the government's nearly four-year  operation.

Calderon sent about 10,000 troops and federal police to Ciudad Juarez in 2008 to fight drug cartels, but killings have surged since then.

More than 200,000 people, mainly wealthy and middle-class residents, have fled the city of about 1.5 million people.

Source: Agencies