The banner appeared over the weekend in Nuevo Laredo near the border with Texas: "Operative group 'The Zetas' wants you, soldier or ex-soldier. We offer a good salary, food and benefits for your family. Don't suffer anymore mistreatment and don't go hungry."
The Zetas is the enforcement arm of the Gulf cartel and is made up of former Mexican soldiers. Photos of the banner were displayed prominently in Mexico's national media on Monday.
An official of the federal attorney general's office, who spoke on condition of anonymity because he was not authorized to give his name, told The Associated Press that authorities believe the banner is authentic, although they are still investigating.
He said officials believe the banner may be a response to the government after it posted signs in several cities with the photos of three Zeta members, asking people to call police if they see them.
In Nuevo Laredo, those signs were recently defaced and several had a telephone number written across them, the official said. The same telephone number appeared on the banner. The Associated Press called the number, but a recording said the number does not exist.
Security expert Jose Luis Pineyro said the recruiting effort could be drug traffickers' way of thumbing their noses at the federal government, but is unlikely to be an effective way of bringing in new blood.
"On the contrary, those who have decided to desert or remain inside the armed forces as spies are already doing that," Pineyro said.
Earlier this month, law enforcement officials said drug cartels were using classified ads to lure young Mexicans in Ciudad Juarez into unknowingly working as drug couriers.
Mexico City's El Universal newspaper first reported the trend, citing ads in Ciudad Juarez newspapers that require applicants to have the U.S. visa needed to drive a vehicle across the border but do not mention job experience.
A federal official, not authorized to give his name, said applicants often think they are applying to become messengers, but they end up unwittingly driving vehicles loaded with drugs into neighboring El Paso, Texas.
Chris Mears, a spokesman for the El Paso Police Department, said he was aware of the trend and had seen several teenagers charged after being found with loads of drugs in their cars.
Tighter border security on both sides has forced drug cartels to find creative ways to move their contraband into the U.S.
Mexican authorities say cartels are training new recruits in the face of President Felipe Calderon's nationwide crackdown. Since taking office in 2006, Calderon has sent more than 20,000 troops to areas plagued by drug violence.