Twenty people died in a prison fight on Wednesday between rival drug gang factions near a Mexican border city, where thousands of troops are battling for control of the streets from cartels.

Members of a small drug gang known as the "Aztecas" seized a guard's keys after visiting hours at the state penitentiary in the desert near Ciudad Juarez. They then opened cell doors, freeing some 170 inmates who went on a rampage.

"They attacked other prisoners in a high security area with iron bars and home-made firearms," said Victor Valencia, the state government representative in the city.

Ciudad Juarez, just across the border from El Paso, Texas, and home to foreign-owned factories that export to the United States, has become the main flashpoint in President Felipe Calderon's two-year-old war against drug smugglers.

Thousands of Mexicans have been killed, including 250 in Ciudad Juarez last month, in the cartels' fight for control of turf and trafficking routes to the United States.

Attorney General Eduardo Medina Mora told Reuters on Tuesday the government was deeply worried about the killings in Ciudad Juarez. U.S. officials have also expressed concern about the lawlessness.

The U.S. and Canadian governments have warned tourists to stay away from dangerous border cities this spring. Mexico traditionally is a prime destination for college students traveling on spring-break vacations.

The Mexican government is sending thousands of extra troops into Ciudad Juarez this week to clamp down on the violence. The cartels, however, are powerful, well-armed and aided by corrupt police.

Mexico's army hopes to have 7,500 soldiers and federal police in the city by the end of this week, patrolling the streets and staffing checkpoints at the airport and on bridges across the Rio Grande into Texas.

About 100 masked federal police and troops in Humvee military vehicles surrounded the prison on Wednesday, and two army helicopters clattered overhead.

Watch the VIDEO: Scores dead in Mexico prison riot