WASHINGTON – A US Immigration and Customs Enforcement agent was shot and killed "in the line of duty" in Mexico and a second was wounded in the attack by "unknown assailants," a top US official said.

Homeland Security Secretary Janet Napolitano said she was "deeply saddened by the news" that the ICE special agents based in the Mexican capital came under fire, with one dying from his injuries.

The pair were "were shot in the line of duty while driving between Mexico City and Monterrey by unknown assailants," she said in a statement.

The second agent was "shot in the arm and leg and remains in stable condition," she added.

"US law enforcement agencies are working closely with Mexican authorities who are investigating the shooting to ensure the perpetrators of this unconscionable crime are captured as quickly as possible," said the statement.

The shooting took place amid a surge in violence in recent days in the country ravaged by turf battles between drug cartels and a four-year-old crackdown by the army on the drug trade.

More than 34,600 people have died in drug violence since December 2006, when President Felipe Calderon deployed soldiers and federal police in a widespread crackdown on the illegal cartels.

"Let me be clear: any act of violence against our ICE personnel -- or any DHS personnel -- is an attack against all those who serve our nation and put their lives at risk for our safety," Napolitano said.

"The full resources of our department are at the disposal of our Mexican partners in this investigation. We remain committed in our broader support for Mexico's efforts to combat violence within its borders."

It was unclear whether the agents were specifically targeted because they worked for the US government.

Amid the latest cycle of violence in the region, 18 people were killed in a series of attacks on a bus, a private car and public buildings on Monday in business hub Monterrey.

The area is the scene of a turf battle between the Gulf Cartel and the Zetas, who are fighting for control of lucrative drug trade routes to the United States.

In the state of Nuevo Leon, attackers blew up a car in which the state intelligence chief was riding late Sunday.