Mexico City  - The dreaded Los Aztecas gang demonstrated this week that it can operate with impunity and kill at will, even within prison walls.

The gang was responsible for the deaths of 20 inmates in a prison in Ciudad Juarez, Mexico, a city that has become a bloody battleground for powerful drug cartels that fight each other and the authorities.

"It is one of the most powerful street gangs on the US-Mexican border," Marco Antonio Romero, the Public Security Ministry spokesman of the northern Mexican state of Chihuahua, told Deutsche Presse- Agentur dpa.

"They do not just work in Ciudad Juarez. They have branches in several US states - Texas, New Mexico, California, Arizona and other states further in."

The gang struck Wednesday at the State Social Readaptation Centre, about 30 kilometres south of Ciudad Juarez. It was early morning and time for marital visits to end. As the inmates said their goodbyes, all hell broke loose. About 100-150 members of Los Aztecas clashed with rival gangs Mexicles and Artistas Asesinos, also known as Doble A.

The jail, which opened four years ago and has a capacity of 1,000 inmates but currently holds about 730, is regarded as a model prison in Mexico and permits all-night marital visits.

According to the authorities, Los Aztecas' members threatened the guards and forced them to hand over the keys to a high-security cell in which their rivals were being held, attacking them with their hands and sticks. They yanked off doors and metal bars to use as weapons.

Prison director Oscar Sergio Hermosillo told Juarez daily Norte that a Los Aztecas member approached a guard when they had finished plundering and killing.

"It's over now, we already achieved our goal," the prisoner said. The gang members then handed over their weapons and returned to their cells.

According to the FBI and police in the US city of El Paso, Texas, across the border from Ciudad Juarez, the gang was created by Mexican Longo Fernandez in 1986 in the US to protect and unite Hispanic residents in the region.

Today, Los Aztecas also operates inside prisons in Mexico and the United States, where they control the sale of drugs and prostitution.

Most of the original members came from the southern El Paso neighbourhood of Segundo Barrio. As Los Aztecas grew, it got involved in drugs, extortion, murder, robbery and intimidation.

It has established alliances with drug cartels and is linked to the formidable Cartel de Juarez, for which Los Aztecas "freelances" - selling drugs and as hired hitmen.

FBI Director Robert S Mueller said last August that the greatest threat in El Paso came from Los Aztecas, and he warned that the gang was starting to migrate to other states.

The gangsters think of themselves as Aztec warriors, and are often adorned with feathered head-dresses and tattoos of snakes and other Aztec symbols, Mexican daily Excelsior reported, describing them as "merciless."

There are reportedly more than 5,000 members of the gang in Ciudad Juarez.

In a travel warning issued in February, the US State Department warned that the "situation in Ciudad Juarez is of special concern."

"Mexican authorities report that more than 1,800 people have been killed in the city since January 2008. Additionally, this city of 1.6 million people experienced more than 17,000 car thefts and 1,650 carjackings in 2008." (dpa)