UPDATE, 1:00p ET: Pentagon spokesman Rear Adm. John Kirby briefed reporters this afternoon, saying the attack occurred during a routine visit by officers to the training facility.

Kirby said the attacker had gone through a "serious vetting process."

Kirby said ISAF has worked hard to "mitigate" the threat of these attacks, but called Afghanistan a war zone where the threat cannot be "completely eliminated." He pointed out that insider attacks have been reduced in the past few years.

He said he believes the two-star general is the highest-ranking American military officer to be killed in Afghanistan, but could not confirm that completely.

UPDATE, 11:50a ET: New information coming in on an insider attack on American troops in Afghanistan, which has reportedly killed a U.S. major general. Read more below from FoxNews.com:

A US major general was shot and killed Tuesday by a man dressed in an Afghan army uniform who opened fire at an army training academy outside Kabul, the Associated Press reported.

At least 15 soldiers -- reportedly mostly Americans and some Germans --were wounded in the close-range assassination-style attack.

Details about the attack at Camp Qargha, a base west of the capital, Kabul, weren't immediately clear. Gen. Mohammmad Zahir Azimi, a spokesman for Afghanistan's Defense Ministry, said a "terrorist in an army uniform" opened fire on both local and international troops. Azimi said the shooter had been killed and that three Afghan army officers were wounded.

A U.S. official told The Associated Press that one American soldier was killed and "about a dozen" of the wounded were Americans, but declined to comment further. The official spoke on condition of anonymity because the official was not authorized to discuss details of the attack by name on the record.

A senior U.S. military official told Fox News that one of the dead was an ISAF service member and the shooting left a "significant number of wounded, both Americans and Afghans." The wounded also included a German brigadier general, who the German military said was receiving medical treatment and was "not in a life-threatening condition."

Germany's military said one NATO soldier was killed, while 15 NATO soldiers were wounded in an assault launched "probably by internal attackers."

In a statement, NATO said that it was "in the process of assessing the situation."

Qargha is known as "Sandhurst in the sand," as British forces oversaw building the officer school and its training program. In a statement, the British Defense Ministry said it was investigating the incident and that "it would be inappropriate to comment further at this time."

The attack comes as so-called "insider attacks" — incidents in which Afghan security turn on their NATO partners — largely dropped last year. In 2013, there were 16 deaths in 10 separate attacks. In 2012, such attacks killed 53 coalition troops in 38 separate attacks.

Such "insider attacks" are sometimes claimed by the Taliban insurgency as proof of their infiltration. Others are attributed to personal disputes or resentment by Afghans who have soured on the continued international presence in their country more than a dozen years after the fall of the Taliban's ultra-conservative Islamic regime.

Foreign aid workers, contractors and other civilians in Afghanistan are increasingly becoming targets of violence as the U.S.-led military coalition continues a withdrawal to be complete by the end of the year.

Greg Palkot reported on a new "insider" attack on American troops in Afghanistan. About a dozen victims of the attack are Americans, with at least one killed, Associated Press reported.

He said an attacker dressed in an Afghan military uniform opened fire on "high-ranking" NATO soldiers, including from the U.S. and Germany.

Palkot pointed out that while reporting from Afghanistan in recent years, he has heard U.S. service members express fears about some of their Afghan counterparts and about infiltration by the Taliban.