The four were among 13 people slain Friday in Guerrero, which has seen a spike in violence since rival factions of the Beltran Leyva cartel began fighting over territory after leader Arturo Beltran Leyva died in a battle with Mexican marines in December 2009.

Four men with their hands and feet tied and heads covered in duct tape were thrown 600 feet to their deaths from a bridge on Friday, authorities said as Mexico?s increasingly bloody drug battles reached a new level of cruelty and intimidation.

The four were among 13 people slain Friday in Guerrero, which has seen a spike in violence since rival factions of the Beltran Leyva cartel began fighting over territory after leader Arturo Beltran Leyva died in a battle with Mexican marines in December 2009.

The other nine were killed in the resort city of Acapulco. In the most gruesome of those killings, police found a severed head that had been scalped and whose face had been skinned.

The unidentified men were dropped from a 600?feet?high (200?meter) bridge near the Guerrero state capital of Chilpancingo, the Guerrero state Public Safety Department said.

The men had bruises all over their bodies and ?it?s presumed they were thrown alive from the Solidarity bridge,? the statement said.

Drug gang members have taken increasingly drastic measures seeking to intimidate rivals, from beheadings to skinning their victims.

Guerrero state authorities said earlier on Friday that eight people, including four teenagers, were slain before dawn in a string of attacks throughout Acapulco. Guerrero state police said it was not clear if the attacks were related.

Nationwide, nearly 35,000 people have been killed in drug?gang violence since President Felipe Calderon deployed troops and federal police four years ago to crush the cartels in their strongholds.

In Mexico?s north on Friday, soldiers killed eight suspected drug cartel members in two clashes near the industrial city of Monterrey, the military said.

Soldiers intercepted a group of gunmen toting high?powered rifles and a grenade launcher and chased them into the Monterrey suburb of Guadalupe, where a shootout left five gunmen dead, Mexico?s Defence Department said in a statement.

Another group of gunmen later fired on soldiers in Juarez, another Monterrey suburb, sparking a firefight that killed three attackers, it said.

Monterrey and the surrounding area has suffered a surge in drug violence as the Gulf Cartel battles the Zetas group for territory.

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