Drug hitmen tortured and killed four men, wrapping their heads in black garbage bags, as thousands of soldiers and federal police arrived to bolster security in the Mexican border city of Ciudad Juarez, local authorities said on Monday.

The bodies were badly burned, handcuffed and half-naked, shot between Sunday night and Monday morning and dumped on the street in different parts of the city, said the attorney general's office for northern Chihuahua state.

The murders came as heavily armed soldiers and police set up roadblocks and raided houses over the weekend in the rundown city across the U.S. border from El Paso, Texas, the start of a 2,500-troop deployment aimed at crushing drug gangs.

Ciudad Juarez, which has drawn worldwide attention because of a rash of brutal murders of women, has had 200 people killed in drug-related violence this year -- 10 times as many as a year ago.

City Hall said soldiers also arrested six local police officers in possession of large quantities of marijuana and blamed police corruption for drug cartels' hold on Ciudad Juarez, a major narcotics smuggling point into the United States.

"People have lost confidence in the police because they know the local forces have been infiltrated and act outside the law," said Mayor Jose Reyes Ferriz.

Local city police have also been stripped of their cellular phones to avoid easy communication with drug gangs, he added.

The troop convoys in Ciudad Juarez are opening up a new front in President Felipe Calderon's 15-month-old war on drug cartels.

Already about 25,000 soldiers and federal police have deployed in hotspots across Mexico, especially along the U.S. border.

The overall death toll associated with drug gangs in Mexico has rise to more than 720 this year, well above the count this time last year.

Mexico's drug wars killed more than 2,500 people in 2007.