Three U.S. Marines were injured in a shooting south of Mexico City on Friday morning, a Mexican military official told CNN Mexico.

The motive behind the attack on the American servicemen, who were on a diplomatic mission, was not immediately known.

Unknown gunmen inside what was described as a Mexican federal police vehicle fired upon a U.S. Embassy vehicle that carried the U.S. servicemen, said the official, who declined to be named for security reasons.

The injured Marines were transported to a hospital in Cuernavaca, Mexico.