Three U.S. military members were killed outside a military base in southern Jordan, U.S. officials said.
An American official in Washington said one U.S. service member died at the scene and two who were wounded died at a hospital in Amman, the Jordanian capital. The official spoke on condition of anonymity because details of how the attack happened were still being investigated.
Jordan is a key U.S. ally and member of a U.S-led military coalition fighting the extremist Islamic State group, which controls parts of neighboring Iraq and Syria.
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Shots were fired as a car carrying the Americans tried to enter the al-Jafr base near the southern Jordanian town of Mann at about noon local time on Friday, military officials in the U.S. and Jordan said. The American official said the U.S. government was working with Jordanian government officials to gather additional details.
In addition to the U.S. casualties, a Jordanian officer was also wounded, Jordanian officials said.
It was not immediately clear what prompted the shooting.
Jordan faces homegrown extremism, with hundreds of Jordanians fighting alongside IS militants in Iraq and Syria and several thousand more supporting the extremist group in the kingdom.
Last November, a Jordanian police captain opened fire in an international police training facility, killing two Americans and three others. The government subsequently portrayed the police captain as troubled.
The United States has spent millions of dollars to help the kingdom fortify its borders. For the West, any sign of instability in Jordan would be of great concern.
Associated Press writer Lolita C. Baldor contributed to this report.