Officials say the pair, described as "special agents" shot "in the line of duty", were driving between Mexico City and northern city of Monterrey.

An official said they had been attacked after stopping at a security checkpoint in San Luis Potosi state.

The attack comes as a US report says a fifth of the Mexico border, some 375 miles (600km), is not properly guarded.

The investigative arm of Congress, the General Accounting Office (GAO), said the area lacked enough border agents to stop illicit activity such as gun running and illegal immigration.