U.S. Ambassador to Mexico Tony Garza (search) told Mexican leaders Wednesday he is concerned that growing drug-related violence and kidnappings on the border between the two countries will have a "chilling" effect on trade and tourism.

Garza's comments in a letter directed to Mexican Foreign Relations Secretary Luis Ernesto Derbez and federal Attorney Rafael Macedo de la Concha explained the motives behind a public announcement from the U.S. State Department to citizens thinking of traveling to the region.

The department on Wednesday alerted Americans visiting Mexico (search) that violent crime, including murder and kidnapping, has increased in its northern border region. The alert attributed the increased crime to a war between criminal organizations struggling for control of the lucrative narcotics trade. It said the leaders of several major criminal organizations have been arrested, creating a power vacuum.

"U.S. citizens should be aware of the risk posed by the deteriorating security situation," the department warned. It added that the great majority of victims are Mexican citizens.

The U.S. Embassy in Mexico distributed Garza's letter to news agencies after issuing the alert.

A Mexican Foreign Relations Department spokesman, Allan Nahum, said officials "have received the notification and we are going to pass it along to the appropriate authorities so that they may respond" as they see fit.

The advisory came just days after the Mexican government sent federal police officers to patrol the streets of the border city of Nuevo Laredo (search), across from Laredo, Texas, at the request of local authorities who said the crime and violence had become too great for them to handle.

It also comes as officials crack down on top-security federal prisons housing reputed drug traffickers who allegedly have continued to operate their businesses from behind bars with the help of corrupt prison employees.

In the latest move against prisons, federal agents and soldiers seized the top-security Matamoros prison, in the border city of the same name across from Brownsville, Texas, after six prison employees were slain and their bodies dumped outside the prison gates last Thursday.

In the latest move against prisons, federal agents and soldiers seized the top-security Wednesday in the border city of Matamoros, across from Brownsville, Texas, after six prison employees were slain and their bodies dumped outside the prison gates last Thursday.

Investigators say the Matamoros prison employees were apparently killed by drug traffickers settling turf disputes and prison vendettas.

Garza noted that while the great majority of border violence victims are Mexican citizens, "the elevated level of violence generally has resulted in greater risks to the thousands of American citizens visiting and passing through the border region every day. Increased numbers of murdered and kidnapped Americans in recent months bear this out."

Garza went on to say that he worried "the inability of local law enforcement to come to grips with rising drug warfare, kidnappings and random street violence will have a chilling effect on the cross-border exchange, tourism and commerce so vital to the region's prosperity.... We certainly do not want at this time to advise Americans to refrain from traveling to Mexico by land or to avoid the border areas, but it is our responsibility to alert them to the enhanced risks," Garza said.

He added that he applauded President Vicente Fox's (search) efforts to combat crime and drug trafficking and said he hoped "that commitment will make a difference at the state and local levels."