Mexico City: A US agent was shot and killed in Mexico Tuesday and a second agent wounded amid a surge in violence in a country ravaged by crime cartel turf wars and a government crackdown on the drug trade.

The US Immigration and Customs Enforcement (ICE) agents were attacked while in a car while driving between Mexico City and the northern industrial Monterrey, US Homeland Security Secretary Janet Napolitano said.

The pair 'were shot in the line of duty' by 'unknown assailants', Napolitano said in a statement.

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

The Mexican Foreign Ministry said it "strongly condemns this grave act of violence," and said that it was working closely with US officials to solve the crime and help the wounded agent.

US embassy officials in Mexico City told AFP that the agents were assigned to an ICE office in the Mexican capital, but did not say if the agents were on an assignment when they were attacked.

Mexican officials did not say where the attack took place, but local TV networks showed footage of a dark colored SUV with tinted glass windows marked with bullet holes on the side of a highway in the central Mexican state of San Luis Potosi.

State officials said that they found a bullet-riddled car with US license plates near the town of Ojo Caliente at 3 pm (2100 GMT) on Tuesday.

A US law enforcement official, speaking on condition of anonymity, told the Dallas Morning News that the agents, who were probably unarmed, were shot as they approached an unofficial checkpoint apparently manned by members of the Zetas paramilitary drug gang.

Drug gangs sometimes set up roadblocks to search for potential kidnap victims, steal cars, or rob travelers.

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.

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

And on Sunday eight people were killed in a gun battle when a special police unit stopped a suspicious vehicle in Monterrey.

Monterrey, a prosperous city and home to the local operations of several multinational corporations, is at the intersection of several highways - often used as drug smuggling routes - heading north into the United States.

Two rival drug organizations, the Gulf cartel and their former allies, the Zetas, are battling for control of the area.

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

ICE director John Morton called the shooting a "stark reminder of the risks confronted and the sacrifices made by our men and women every day."

"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," Napolitano said.

"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," she added.