Three US military trainers were shot dead in Jordan today when the car they were in 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 airbase in the south of the country, a close ally of the United States. A Jordanian army guard was also shot and wounded during the exchange of fire.

Another security source said it was not possible to rule out any political motives in the attack in an airbase that has dozens of US trainers working alongside Jordanians.

“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 clarified.

“An investigation is now under way to know exactly what happened,” the source said.

Jordan hosts several hundred US contractors in a military program to bolster the kingdom’s defences, which includes the stationing of F-16 fighter jets that use Jordanian airfields to hit Daesh positions in neighbouring Syria.

Since the start of the Syrian conflict in 2011, Washington has spent millions of dollars to help Jordan set up an elaborate surveillance system known as the Border Security Programme to stem infiltration by militants from Syria and Iraq.

But Jordan’s role in the war against Daesh has raised disquiet among some Jordanians about instability on their borders.

US officials say aid to Jordan, one of the largest recipients of US foreign military assistance, is expected to rise to $800 million in 2016 and grow in future years.

The last incident involving US personnel was in November last year when a Jordanian officer shot dead two US government security contractors and a South African at a US-funded police training facility near Amman before being gunned down.

The incident embarrassed the Jordanian authorities, who did not publicly disclose the motive of the assassin. Security sources later said that the gunman was a Daesh sympathiser, while others disputed this and said he simply had strong anti-Western feelings.