The Pentagon on Tuesday confirmed that a US general was killed in an attack in Afghanistan — the highest-ranking American fatality since the 9/11 attacks.
The US Defense Department also identified the assailant, who was wearing a uniform, as an Afghan soldier and said that he was killed after he opened fire.
The Pentagon's spokesman Rear Admiral John Kirby said that "up to 15" people were injured in the attack but said that they were not all Americans.
A man in Afghan military uniform opened fire on NATO troops at a British-run officer academy in Kabul on Tuesday, officials said, wounding three Afghans as details of foreign casualties remain unclear.
"We are investigating, but it appears that an Afghan army officer opened fire," General Mohammed Afzal Aman, the chief of staff for operations at the Afghan Ministry of Defense, told AFP.
"Three of our officers have been injured. ISAF (International Security Assistance Force) troops have also suffered casualties."
"ISAF have quarantined the site, allowing nobody, including Afghan forces, to approach."
NATO's International Security Assistance Force said in a statement: "We can confirm that an incident occurred involving local Afghan and ISAF troops at Camp Qargha."
As details of the incident remained scarce, defense ministry spokesman Mohammad Zahir Azimi described the attacker as "a terrorist" — suggesting he may have infiltrated Afghan forces.
"A terrorist wearing Afghan army uniform opened fire at national army officers and their foreign colleagues and wounded several people," Azimi said on Twitter.
"The ministry of defence strongly condemns this attack. The attacker was killed by the Afghan army."
Western officials say that most so-called "insider attacks" stem from personal grudges and cultural misunderstandings rather than Taliban insurgent plots.
Incidents in which Afghan soldiers turn their guns on their allies have killed scores of US-led troops in recent years, breeding fierce mistrust and forcing joint patrols to be protected by armed guards known as "guardian angels."
But the shootings dropped rapidly last year as ISAF troops closed many bases and reduced combat operations before the end of their mission in December.
"We are aware of reports of an incident at Qargha," the British ministry of defence said in a statement issued from London.
"The incident is under investigation and it would be inappropriate to comment further at this time."
Academy for Afghan elite
The Afghan National Army Officer Academy was hit by another "insider attack" last year when an Afghan soldier shot and wounded two NATO coalition troops over a dispute before being killed.
The academy is a flagship training facility that opened in October to produce a new generation of professional military leaders as US-led NATO troops end their war and the Afghan army takes on the Taliban insurgents.
Overseen by British mentors, the academy is loosely modelled on Sandhurst, the renowned British officer training school.
The Afghan military has been built from scratch since the fall of the Taliban regime in 2001, and it has struggled with high casualty rates, "insider attack" killings, mass desertions and equipment shortages.
In February this year, two Afghan men wearing military uniforms shot dead two US soldiers in the eastern province of Kapisa.
Also on Tuesday, Afghan officials accused an ISAF air strike of killing four civilians in the western province of Herat.
"After rockets were fired at Shindand airbase, an aircraft carried out strikes on the area where they were launched," Herat's deputy governor Asiludin Jami told AFP.
"A man, a woman, a kid and a teenager were killed. They were all civilians."
ISAF said it took all allegations of civilian casualties seriously, and was assessing the facts surrounding this incident.