AMMAN - Three US troops were killed in a shooting outside a military training facility in Jordan on Friday, a US official said.

"A total of three US service members died today in the incident in Jordan ," the official said in Washington. "The service members were in vehicles approaching the gate of a Jordanian military training facility, where they came under small arms fire," the official added.

"Initial reports were that one was killed, two injured. The two injured service members were transported to a hospital in Amman, where they died." "We are working with the Jordanian government to gather additional details about what happened."

Earlier, the Jordanian army said the shooting took place at the gate of Al-Jafr base in southern Jordan after the car carrying the US trainers failed to stop. It said that a Jordanian officer was also wounded.

"An exchange of fire occurred Friday morning at the gate of the Prince Feisal Air Base in Al-Jafr when a car carrying trainers attempted to enter the gate without heeding the guards' orders to stop," it said in a statement, quoting a military source.

The army said an investigation was under way to determine the causes of the shooting.

An American defence official described the incident as "green on blue", a military term for when friendly forces attack US personnel.

"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 AFP.

The death of three American troops in Jordan could prove very embarrassing for Amman, a key recipient of US financial aid and member of the US-led coalition fighting the Islamic State group in neighbouring Syria and Iraq.

US forces have trained a small group of vetted Syrian rebels in Jordan , and American instructors have trained Iraqi and Palestinian security forces in Jordan as well over the past few years.

Friday's incident comes almost a year after a Jordanian policeman shot dead two US instructors, a South African and two Jordanians at a police training centre east of Amman, before being gunned down.

Washington said at the time that the two Americans killed in the November 9, 2015 shooting were employees of the private firm DynCorp contracted by the State Department to train Palestinian forces.

Two other Americans were wounded in that incident, which sparked concern in Washington and was condemned by the US embassy.

The centre where last year's shooting took place was set up after the US-led invasion of Iraq in 2003.

It has since trained tens of thousands of Iraqi, Palestinian and Afghani police officers, and last year Jordan announced that former Libyan rebels would also be trained there.

A government source said that military training is provided at Al-Jafr air base by instructors of various nationalities, including Americans, to participants from different countries.

DynCorp International spokeswoman Mary Lawrence said on Friday the firm's employees had all been accounted for after the shooting and that none had been wounded.

Last year, the United States announced its intention to increase overall US assistance to Jordan from $660 million to $1 billion annually for the 2015-2017 period.