Americans traveling and living in Mexico have been put on alert by the U.S. Department of State because of escalating violence, especially in the country's northern border states.

"While millions of U.S. citizens safely visit Mexico each year (including thousands who cross the land border every day for study, tourism or business), violence in the country has increased recently. It is imperative that travelers understand the risks of travel to Mexico, how best to avoid dangerous situations, and whom to contact if one becomes a crime victim," the Bureau of Consular Affairs said in a statement. "Common-sense precautions such as visiting only legitimate business and tourist areas during daylight hours, and avoiding areas where prostitution and drug dealing might occur, can help ensure that travel to Mexico is safe and enjoyable."

"Robberies, homicides, petty thefts, and carjackings have all increased over the last year across Mexico generally, with notable spikes in Tijuana and northern Baja California," the statement said. "Ciudad Juarez, Tijuana and Nogales are among the cities which have recently experienced public shootouts during daylight hours in shopping centers and other public venues. Criminals have followed and harassed U.S. citizens traveling in their vehicles in border areas including Nuevo Laredo, Matamoros, and Tijuana. The situation in Ciudad Juarez is of special concern." (Video: Alert Issued)

Mexico has deployed about 40,000 troops in its war on the drug cartels since December 2006, and insists that the tactic has resulted in record numbers of seizures and arrests. But as the crackdown makes it harder for the cartels to operate, they resort to violence against the troops and each other, with civilians caught in the crossfire.

Among the recent violence:

• Feb. 11: Gun battles between suspected drug gang members and troops in the northern Mexican town of Villa Ahumada, Chihuahua, left 21 dead.
• Jan. 24: A man in police custody calling himself the "stew maker" said in a news conference was paid $600 a week to dissolve the bodies of murdered rival gang members in caustic soda.
• Jan. 7: Masked gunmen opened fire at a television station in the northern city of Monterrey. Since 2006, 15 journalists have been killed in Mexico and many local newspapers have stopped investigative reporting of the drugs cartels.

Texas Gov. Rick Perry, worried about the spread of violence by Mexican drug cartels, said on Tuesday that he wants 1,000 troops to help guard the Texas-Mexico border.

"I don't care if they are military, National Guard or customs agents. We're very concerned that the federal government is not funding border security adequately. We must be ready for any contingency," he said a news conference in El Paso, which sits across the border from Juarez.

In Arizona, the Mexican drug war has had a detrimental effect on businesses, and major universities have advised their students to avoid going to Mexico for spring break this year, Phoenix TV station KPHO reported.

Ciruli Brothers Produce, of Phoenix, imports fruits and vegetables from Mexican farmers, and its owner said it takes twice as long to get from the fields in Sinaloa to his air-conditioned warehouse in Nogales, Ariz., because of the government checkpoints they need to cross.

The Border Patrol has also felt the impact of the violence, and the increased sophistication and boldness the smugglers are using to get their product into the United States.

In one case, agents said they intercepted a smuggling operation that attempted to use a large vehicle carrier as a ramp to drive two cars packed with marijuana over the border wall into the United States.

When border agents confronted the two dozen smugglers they started throwing the bundles of marijuana back over the border wall into Mexico. Shots were fired. Molotov cocktails thrown. And a vehicle was torched before the smugglers crossed back into Mexico, KPHO reported.

On Feb. 21, the University of Arizona in Tucson issued a travel advisory urging students not to go to Mexico, and officials at Arizona State University in Tempe and Northern Arizona University in Flagstaff said they have similar plans to warn students. The schools' spring breaks fall on the second or third weeks of March.

More than 100,000 American teens and people in their early 20s travel to resort areas throughout Mexico during spring break every year, according to the State Department. It maintains a section on its Web site aimed specifically at cautioning college students taking spring break in Mexico, pointing out that while most enjoy the experience, "several may die, hundreds will be arrested, and still more will make mistakes that could affect them for the rest of their lives."

The violence also isn't a factor for University of Arizona sophomore Daniel Wallace. The molecular and cellular biology major told The Associated Press he is driving with seven friends and his father four hours from Tucson to the resort town of Puerto Penasco -- or Rocky Point -- and will spend his entire spring break there.

Alerts such as the one for Mexico address what the department calls "short-term conditions, generally within a particular country, that pose imminent risks to the security of U.S. citizens."

More serious warnings can be issued for "long-term, protracted conditions that make a country dangerous or unstable," and sometimes coincide with U.S. embassies working under restricted conditions or being forced to draw down staff.