 -- Two Americans have been killed and three others were injured in an attack near a coalition base near Kabul, Afghanistan, in what an Afghan official described as an insider attack.

"The two individuals were killed during an attack near a coalition base by an unknown assailant, who was later killed. They were conducting duties as part of the larger NATO mission to train, advise, and assist the Afghan security services. An investigation is being conducted to determine the exact circumstances of the event," according to a statement released by U.S. Forces Afghanistan. "One U.S. service member and two U.S. civilians also sustained wounds and are currently stable."

Dawlat Waziri, a spokesman for the Afghan Defense Ministry, told ABC News that the shootings occurred during an insider attack. According to Waziri, the attacker was wearing an Afghan National Army Uniform and was killed in an exchange of gunfire.

A U.S. official said the incident occurred at an Afghan military ammunition supply point near Camp Morehead, outside of Kabul. According to the official, the Americans were visiting the facility as part of their train, advise and assist mission.

The official said the attacker opened fire on them at the entry control point to the camp.

"Anytime we lose a member of our team, it is deeply painful," said Gen. John W. Nicholson, commander of U.S. Forces-Afghanistan and Resolute Support. "Our sympathies go out to the families, loved ones, and the units of those involved in this incident. To those who continue to target coalition forces, ANDSF, and Afghan civilians, RS and USFOR-A will continue to pursue our train, advise, and assist mission to help our partners create a better Afghanistan."