A US immigration and customs agent has been shot dead and a second wounded in an attack on their car in Mexico.

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

Some reports suggest they were attacked after stopping at what appeared to be a military checkpoint.

US Homeland Security Secretary Janet Napolitano said she was "deeply saddened" by the attack.

"Any act of violence against our personnel [...] is an attack against all those who serve our nation and put their lives at risk for our safety," she said.

She added the US was committed to supporting Mexico's efforts to combat violence within its borders".