A US service member was killed outside a Jordanian training facility on Friday, a US defense official said, after the Jordanian army reported two US trainers were dead.

"One US service member was killed and two others were critically injured just after noon local time in Jordan as they approached the gate to a training facility," the official said.

"The wounded service members were medevac'ed to King Hussein Hospital in Amman."

The Jordanian army statement had originally said that "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."

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

Al Jazeera reports that the US embassy in Jordan has confirmed the incident.

An embassy statement that "We have reports of a security incident involving American personnel and we are in contact with Jordanian officials who are giving us all their support."

Jordan, a key US ally in the Middle East, is a 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 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 sparked concern in Washington and was condemned by the US embassy.

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.

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.