More than 50 federal agents battled alleged drug traffickers in a shootout in the border city of Nuevo Laredo, officials said Saturday, the latest incident in a wave of violence that has sparked criticism from the U.S. government.

Several people were killed in the gunbattle late Friday, said a spokeswoman with the federal attorney general's office. She could not confirm the number and requested anonymity because the agency was waiting to issue a formal statement on the incident.

Officers with the Federal Preventative Police were involved in the shootout, which lasted 40 minutes, according to an official with the agency who also requested anonymity.

On Saturday morning, federal agents in ski masks and body armor and six military humvees packed with soldiers sealed off the scene of the shootout and refused to let journalists enter.

The soldiers refused to comment or confirm if they had been involved in the shooting.

Residents said they had heard constant gun fire and some shots sounded like they could have come from military-caliber weapons. Several houses on the edge of the scene were riddled with bullet holes.

Nuevo Laredo, across the Rio Grande from Laredo, Texas, has been beset with execution-style killings and shootouts as two drug cartels battle for billion-dollar smuggling routes into the United States.

The bloodshed is part of a wave of drug violence across Mexico this year that has claimed the lives of more than 1,500 people and included beheadings and the assassination of several police chiefs.

On Friday alone, there were 14 drug-related killings, in the states of Baja California, Guerrero, Michoacan and Sinaloa, as well as the shootout in Nuevo Laredo, Mexican media reported.

U.S. Ambassador to Mexico Tony Garza urged Mexican authorities to try and bring order to the border area and on Thursday Mexican and U.S. officials met in Laredo.

On Friday, President Vicente Fox said that violent crime is worse in the United States than in Mexico, and the U.S. government should focus on its own problems.