Three U.S. service members were killed Friday in a shooting incident at a military base south of Jordan’s capital city Amman, a U.S. official said.

The service members were in vehicles approaching the gate of a Jordanian military training facility, where they came under small arms fire, the official said. The U.S. was working with Jordanian officials to investigate further.

Jordanian state news agency Petra first said the shooting occurred at the entrance to the King Faisal Air Base, in the al-Jafr area south of Amman, after the vehicles carrying the service members failed to stop at the security gates.

The agency later dropped the reference to a vehicle failing to stop at a gate, saying only that there was an “exchange” of gunfire.

A Jordanian military source reiterated to CBS News, however, that the incident occurred at a gate to the base, when a vehicle carrying the trainers failed to stop.

The Jordanian source told CBS News it appeared to be an accidental shooting rather than terrorism, but the nature of the incident remained unclear.

The U.S. Embassy in Jordan said in a statement that it was aware of “reports about a security incident involving U.S. personnel” and that it was in contact with Jordanian authorities, “who have offered their full support.”

Attacks on U.S. personnel are extremely rare in Jordan, a close American ally in the Middle East. Jordan is one of the only Arab nations to have joined the U.S.-led military coalition fighting ISIS in Iraq and Syria.

In November 2015, a Jordanian police officer opened fire at a regional police training center in the Jordanian capital, killing two Americans, a South African and two Jordanians before being shot dead.