At least 13 people have been killed in seven attacks in the embattled Mexican border city of Ciudad Juarez, authorities have said.

Gunmen killed six men in four separate attacks on Tuesday in the city across the border from El Paso, Texas.

Two of the victims - a former municipal policeman and a man - were shot dead inside a car, investigators said.

Hours later, assailants killed five young men aged between 16 and 20 outside a home, a Chihuahua state police spokesman said.

Two more men were fatally shot in separate attacks on Tuesday night, police said.

The killings resembled organised crime hits, but police gave no more information about motives or suspects - and no arrests have been made.

On Wednesday, police were still investigating at the scene of the various attacks.

Rival smugglers

With more than 800 people killed this year, Ciudad Juarez has the country's highest level of drug-related violence.

The government says most of the killings are the result of rival smugglers battling for control of lucrative routes into the US.

Drug violence in Mexico has killed more than 11,000 people nationwide since December 2006.

On Tuesday, the Mexican army and prosecutors said they have started destroying over 79,000 firearms seized a decade or more ago.

The weapons are being destroyed at a military base in Mexico City.

The defence department said it would retain a total of 35,372 firearms seized since Felipe Calderon, the Mexican president, took office in 2006, saying those guns were evidence in criminal investigations.