KABUL - Two Americans were killed and another three were injured in a rare attack on foreign troops in the Afghan capital Wednesday, U.S. and Afghan officials said.

A gunman fired on international advisers at an ammunition depot near Camp Morehead, a training site for Afghan commandos, about six miles south of Kabul.

The attack, which took place near the entrance of the base, killed one U.S. service member and injured another. One U.S. civilian was also killed, and two more were wounded in the assault, a statement from the NATO-led coalition said.

The gunman, which the Afghan Defense Ministry said was wearing an Afghan army uniform, was killed after international troops responded with gunfire. The injured Americans remained in stable condition, NATO said.

"It happened near an arms depot. The attacker opened fire on his [coalition] friends," Defense Ministry spokesman Dawlat Waziri said.

A U.S. defense official, speaking on the condition of anonymity because the situation was still developing, said the Americans were at the depot as part of the NATO training mission for Afghan security forces.

The United States supplies the NATO mission with about 6,800 troops to advise and assist Afghan troops, which are battling a fierce Taliban insurgency in key areas across the country. An additional 3,000 U.S. troops are dedicated Operation Freedom Sentinel, a separate counterterrorism mission focused on al-Qaeda and the Islamic State.

The U.S. official said it was not clear Wednesday whether the assailant was in fact a member of the Afghan National Army. But Wednesday's shooting appeared to be what is known as an "insider" or green-on-blue attack, where Afghan allies turn on their U.S. and foreign mentors.

At least 150 coalition troops have been killed in such attacks since 2008, according to data compiled by the Long War Journal, which is published by the D.C.-based Foundation for the Defense of Democracies.

The assaults - which have been attributed both to Taliban infiltration and cultural differences between Afghan and foreign troops - surged in number from 2011 to 2013. But the attacks became more infrequent as the bulk of U.S. and NATO troops withdrew from the country. U.S. forces have suffered only two combat casualties in Afghanistan this year, both of which occurred in the volatile Helmand province in the country's south.

At Camp Morehead, elite Afghan commando units learn ambush tactics, how to call in airstrikes, and train for short missions. But the commandos, which number about 11,000, are increasingly stretched thin.

As Afghan forces struggle to beat back Taliban militants in places like Lashkar Gah and Kunduz, which was briefly taken by insurgents again this month, casualties and desertions have depleted their ranks.

"Anytime we lose a member of our team, it is deeply painful," Gen. John W. Nicholson, commander of U.S. forces in Afghanistan, said Wednesday in a statement. There were no further details on the two Americans who were killed.

More than 2,350 U.S. troops have died supporting military operations in Afghanistan since 2001.