"We in the news media should condemn and reject the violence arising from organized crime," the agreement says.

It also vows to "ignore and reject any information coming from criminal groups with the purpose of propaganda."

Mexican drug cartels frequently leave messages or banners next to the bodies of their victims, often with misspelled, obscene threats to authorities or rival gangs, and some media outlets in Mexico already have a policy of not reporting those messages.

But some of the messages have proven newsworthy.

In July 2010, the director of a prison in northern Mexico was charged with allowing inmates allied with the Sinaloa cartel to make forays out of the prison to murder their rivals. Prison officials allegedly even lent the inmates guns and vehicles.

The Zetas gang, a rival of the Sinaloa cartel, first drew attention to the scandal by kidnapping a local police officer and forcing him to describe the scheme on a video posted to a website that specializes in drug underworld information.

While supporters of the accord denied that the government had been involved in drawing up the guidelines, President Felipe Calderon's office praised the accord as "a clear example of the responsible way in which the participating media outlets treat criminal organizations and the violence they create."

The accord was signed last week by officials of Mexico's two dominant TV networks, a number of large radio station chains and several of Mexico's most influential newspaper groups.

But some organizations declined to sign on to the list of self-imposed rules, including prominent newspapers such as Reforma and La Jornada.

Reforma issued a statement saying that it "has had its own mechanisms for editorial policy."

Benoit Hervieu, the head of the Americas desk for Paris-based Reporters Without Borders, said his group had been invited to sign, but declined because of concerns about some articles, especially one that reads "When the government takes action within the limits of the law, it should be made clear that the violence is caused by the criminal groups."

"I do not totally agree with that," Mr. Hervieu said. "When there are raids, even when they are within the limits of the law, there can still be abuses, even against the news media."

Police and soldiers have sometimes shoved, beaten or threatened reporters covering raids.

Another clause says journalists "should avoid [using] the language and terminology used by criminals," and pledges "as news media, we should not publish information that places at risk the viability of actions and raids against organized crime, or place at risk the lives of those who fight it, or their families."

In 2009, assailants gunned down the mother, aunt and siblings of a marine who died during a raid that killed Arturo Beltran Leyva, the leader of one of Mexico's most powerful cartels. The federal government had released the marine's name, something it no longer does.

Carlos Lauria, senior coordinator of the New York-based Committee to Protect Journalists, said that none of the clauses seem to attack freedom of expression and that if some organizations object to some of the guidelines, "if it generates a debate, it seems to me that would be positive."

"We couldn't have a worse situation than the one we have today," Mr. Lauria said, referring to the threats, violence and attacks by drug gangs that have led some newspapers in northern Mexico to stop publishing articles on drug gang turf battles.

The accord aims to defend journalists' safety, suggesting possible measures such as omitting bylines on some stories. It also defends the media's right to criticize government anti-crime actions, and questions the police practice of displaying newly arrested suspects, often surrounded by their alleged weapons, before reporters and cameras.

"The authorities sometimes try to show how well they are doing in the fight against organized crime by parading suspects before the media in conditions that violate the presumption of innocence," the agreement says. "We as media should always publish this type of information based on the assumption they are innocent until proven guilty."

News anchor Joaquin Lopez Doriga of the Televisa network denied that the rules amount to self-censorship.

"The organized crime gangs very clearly want to use some of us as their spokesmen," Mr. Lopez Doriga said, adding that he had never broadcast a drug cartel message.

Milenio television news director Ciro Gomez Leyva said he would still broadcast drug cartel banners "when those messages are newsworthy. News will continue to be news."