Three US troops were killed in a shooting outside a military training facility in Jordan on Friday, the Pentagon said.

An initial report showed they came under fire as they were entering the facility in vehicles, Pentagon press secretary Peter Cook said.

"We are working closely with the government of Jordan to determine exactly what happened," he said in a statement.

The Jordanian army had earlier 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.

A Jordanian officer was also wounded, it added.

"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.

Two US officials confirmed to AFP that the Americans had also fired their weapons.

An investigation was under way to determine the causes of the shooting, the army said.

Initial reports put the death toll at one or two before the trainers succumbed to their wounds.

An American defense 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.

White House press secretary Josh Earnest said the US troops were part of an "ongoing" training mission.

"The United States is quite interested in getting to the bottom of what occurred," he said.

- Potentially embarrassing -

The death of American troops in Jordan could prove highly embarrassing for Amman, a key recipient of US financial aid and member of the US-led coalition fighting the Islamic State group in neighboring 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 center 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 prompted concern in Washington and was condemned by the US embassy.

The center 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 Jordan announced last year that former Libyan rebels would also be trained there.

Instructors of various nationalities, including Americans, provide training at the base to participants from different countries, a government source said.

DynCorp International spokeswoman Mary Lawrence said on Friday that 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.