"The Zetas have been trying to wage war on everybody for a while"
US Drug Enforcement Agency
"It's an issue of a common enemy," Will Glaspy, the head of the DEA's office in the Texas border town of McAllen, said.
"The Zetas have been trying to wage war on everybody for a while," he said.
"It's been well-documented that the Gulf cartel has formed alliances with the Sinaloa cartel and [La Familia] to wage war against the Zetas."
The pact is believed to have followed a break between the Gulf cartel and the Zetas gang, which was first set up in the 1990s as a group of special forces hit men for the Gulf but quickly began to rival its creator.
Intelligence agencies say the reach of the Zetas now extends to Central America, where they have corrupted police and set up training camps in Guatemala.
Speculation over a pact between the cartels to take on the Zetas has been growing since earlier this year when banners began appearing across the region announcing the campaign by "the cartels of Mexico united against the Zetas".
One banner even urged Mexico's president to withdraw the army and let the new alliance exterminate the Zetas.
At the same time videos and emails have been distributed warning families to stay at home, saying the conflict would get worse.
Police have said the new alliance is fuelling violence in northwestern Mexico, where heavily armed drug gangs have attacked army positions, and added a new front to Mexico's increasingly bloody drug war.
More than 18,000 people have died since 2006 when Felipe Calderon, the Mexican president, began the deployment of around 50,000 troops to regions along the US border and other areas in an effort to crack down on drug trafficking gangs.