Three U.S. military trainers were shot dead in Jordan on Friday when their car failed to stop at the gate of a military base and was fired on by Jordanian security forces, a Jordanian military source said.

The incident occurred at the Prince Faisal air base in the south of Jordan, which is a close strategic ally of the United States. Two trainers died immediately and the third later in a hospital. A Jordanian army guard was also shot and wounded.

"There was an exchange of fire at the entrance to the base after an attempt by the trainers' vehicle to enter the gate without heeding orders of the guards to stop," the military source said. "An investigation is now under way to know exactly what happened."