Powerful drug cartels are increasingly using gruesome videos of executions and interrogations to intimidate everyone from their rivals to police - and an already terrified public - in Mexico's vicious drug war.

The drug gangs, battling for control of lucrative smuggling routes into the United States, have long attached handwritten notes to victims they dump in public as a way to scare rival gangs and pesky state officials.

But they are now airing more and more of their dirty work and threats on blogs or websites like YouTube, and bullying Mexican media into putting their gory tapes on television for wider play.

The aggressive media strategy raises new questions about whether President Felipe Calderon's war on drugs is making any headway in weakening the gangs and in reining in a drug trade worth up to $40 billion a year in Mexico alone.

About 28,000 people have been killed since Calderon took office in late 2006 and sent thousands of federal police and troops to crack down on the drug cartels.

While the gangs began using the bloody videos to send a message to police and rivals several years ago, they have become more common in recent months as turf wars escalate across the country, experts and police say.

The format of the tapes is often the same: captives, many bloodied from beatings, are tied up, blindfolded and posed in front of a draped sheet in an anonymous setting.

Surrounded by heavily armed captors in ski-masks and guided by questioning from an off-camera voice, the captives are forced to confess allegiances to cartels or corrupt officials. Many are then murdered on-camera.

The most explicit videos, when detected, are usually removed by major web sites like YouTube but stay posted on "narco-blogs" run by anonymous administrators.

In one video, a man with a black eye is tethered to a chair in his underwear and appears to be strangled to death with a tourniquet by his captors. There is a "Z" scrawled across his chest, for Zetas, a spinoff of the Gulf Cartel.

In another video, a man is slowly beheaded with a knife.

IMPUNITY, FREE PUBLICITY

"The message is always the same: be afraid," said Maria Guadalupe Licea, head of the government prosecutor's office in Baja California state, home to the violent city Tijuana.

Licea said the use of new technologies and media is part of a spiraling cycle of violence in which ever more shocking attacks inspire copycat killings.

Authorities say that the videos, while graphic, are often little use in tracking down drug gangs.

A spokesman for Mexico's federal police said the apparent murders cannot be investigated until an official complaint is raised and there is no way to prove the videos are real.

No one has been arrested yet for posting the videos.

There are signs the cartels are taking their propaganda war to a new level.

In a frightening development last week, traffickers kidnapped four journalists in northern Mexico and pressured their employers to air 15 minutes of videos showing allegedly corrupt police and rivals being interrogated as a condition for release.

"It is free publicity. A company has to pay thousands of pesos to promote their products. But for these criminal groups ... it just takes a threatening telephone call or grenades chucked at a television station," said Javier Oliva, a security expert at Mexico's National Autonomous University.

The push for the spotlight poses a new danger for journalists in Mexico, already one of the world's most dangerous countries for reporters.

More than 30 journalists have been killed here since Calderon took power.

While the kidnapped journalists were still being held, TV anchors aired public messages condemning the seizures, and Mexico's largest broadcaster Televisa cancelled a news programmes in protest.

"The seizure of our peers and colleagues represents in a wider sense the hijacking of the entire media," Televisa host Denise Maerker said on air. "The risk is that in the future many other media outlets will find themselves in this same delicate situation."