Jordanian military units take part in a parade in June. (Muhammad Hamed / Pool/EPA)

At least three U.S. military trainers in Jordan were fatally shot by security forces Friday when their vehicle failed to stop at the gate of a military base, Jordanian and U.S. officials said.

The U.S. military service members “came under fire” as they approached a Jordanian training facility, said Pentagon press secretary Peter Cook. Few other details about the incident were given. Cook said U.S. and Jordanian authorities were investigating the cause.

Earlier, Jordan’s military said there was “an exchange of gunfire” after the vehicle’s driver ignored demands to stop outside an air base in southern Jordan. A Jordanian officer was injured, the statement said.

Other details of the incident at the King Faisal Air Base were not immediately clear. Investigators were trying to piece together the events, including whether a possible miscommunication was to blame.

The U.S. official said two of the service members died later in Jordan’s capital, Amman, where they were airlifted for treatment.

Jordan is a close ally of the United States, and military training by U.S. personnel is common. Jordan is also part of the U.S.-led coalition fighting the Islamic State in neighboring Syria.

The U.S. Embassy in Amman said in a statement it was “in contact with the appropriate Jordanian authorities, who have offered their full support.”

A U.S. diplomat in Amman, who spoke on the condition of anonymity because he was not authorized to brief reporters, said American and Jordanian officials “do not believe” the incident was terrorism-related, but he offered no further details.

The King Faisal base, about 150 miles southeast of Amman and near the border with Saudi Arabia, has long been used for joint exercises between Jordan and its various allies, including the United States, Britain and Saudi Arabia. The base is also part of the network in Jordan to train Western-backed Syrian rebels.

According to U.S. diplomats and Jordanian officials, there are more than 1,000 U.S. military personnel based in Jordan, the majority serving as advisers to Jordan’s armed forces and Syrian rebel factions.

Jordan’s military has been on high alert since June, when a suicide attacker driving a bomb-rigged truck barreled through Syria’s border with Jordan, setting off a blast outside a Syrian refugee camp that killed seven Jordanian troops. It was the deadliest attack along the tense border, which hundreds of thousands of Syrians have crossed during the more than five-year conflict in their homeland.

In November 2015, a Jordanian police officer opened fire at a police training academy south of Amman, killing two U.S. contractors.

Murphy reported from Washington. Missy Ryan and Thomas Gibbons-Neff in Washington contributed to this report.