Three US servicemen were shot dead during a firefight at an air base in Jordan, according to Department of Defense officials.
The service members were on a training mission, and the incident occurred as they drove their vehicles into the al-Jafr facility. One was killed at the base and two others died later in hospital.
An earlier statement from the Jordanian military said the car they were in failed to stop at the gate and was fired upon by security forces.
"A total of three US service members died today in the incident in Jordan," the official said. "Initial reports were that one was killed, two injured. The two injured service members were transported to a hospital in Amman, where they died."
AFP news agency quoted a U.S. defense official as saying it was a "green on blue" incident, meaning that it may have been friendly fire. "But we can't say for the moment if it was a deliberate" act to kill US personnel or "some kind of misunderstanding," the official told the news agency.
Jordan is a key U.S. ally in the Middle East and has been helping train rebels in Syria to fight the Syrian government.