MEXICO CITY - Gunmen shot dead a US customs agent and wounded another on Tuesday as they drove along Mexico's main north-south highway to Monterrey on official business.

US authorities condemned the attack, which came just over two weeks after US Homeland Security Secretary Janet Napolitano warned Mexico's powerful drug cartels not to take their violent tactics across the border.

Related readings:  7 dead, 1 injured in suburban Mexico City shootout  10 gunmen killed in clash with soldiers in Mexico  Mayor of town in south Mexico shot dead  Fifteen beheaded bodies found in Mexico

US officials did not say why the two Immigration and Customs Enforcement (ICE) agents were going to Monterrey.

"Any act of violence against our ICE personnel ... is an attack against all those who serve our nation and put their lives at risk for our safety," Napolitano said in a statement.

The shooting occurred in mid-afternoon south of the city of San Luis Potosi, which is roughly half way between Mexico City and Monterrey, the country's business capital where drug-related violence has soared in recent months.

Television footage showed a blue sports utility vehicle with several large bullet holes lying in the median of the highway, which was guarded by heavily armed Mexican federal police.

More than 15,000 deaths were blamed on drug violence in Mexico last year but, despite growing domestic criticism of President Felipe Calderon's army-led strategy, the government has vowed to press on with its campaign to crush the cartels.

The violence has alarmed Washington, which worries the fighting could spill over the border. It has also prompted some companies to reconsider plans to invest in Mexico.

The United States has provided funds and training to help Mexico in its fight with the cartels and intelligence from US law enforcement sources is credited with helping Mexico kill and capture several cartel leaders in recent years.