Two U.S. service members were killed and six more wounded during an attack on a joint operation between U.S. and Afghan forces on Saturday in Eastern Afghanistan.

"Upon completing a key-leader engagement at the district center, current reports indicate an individual in an Afghan uniform opened fire on the combined U.S. and Afghan force with a machine gun," Col. Sonny Leggett, a spokesman for U.S. military in Afghanistan, said in a statement.

It remains unknown if the gunman was actually an Afghan soldier or a person who had stolen a military uniform.

Leggett said the cause and motive behind the attack are unknown and that the incident is under investigation.

Names of the service members killed will not be released until 24 hours after their next of kin are notified. The wounded service members are being treated at a U.S. facility.

The attack occurred in the Sherzad district of Afghanistan's Nangarhar province.

Afghanistan's Ministry of Defense said in a statement on Sunday that one Afghan soldier was killed in the attack.

"As a result, unfortunately, one brave ANA member was martyred and three brave members injured; and two brave Resolute support members lost their lives and six brave members were injured," Afghanistan's Ministry of Defense said in a statement.

As NPR's Jennifer Glasse reports, these green on blue attacks, in which Afghan forces or infiltrators turn their weapons on U.S. and coalition troops, were a problem for several years, with the number of attacks peaking in 2012. At that time, they accounted for 15% of all coalition deaths.

In its statement, the Afghan Ministry of Defense said, "Incidents such as this fail to have negative effects on the friendship & spirit of cooperation & between the ANDSF and U.S Military forces. We will continue our fight against terrorism together."

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