Kabul:
An Afghan soldier opened fire on NATO troops at a British-run military academy in Kabul on Tuesday, officials said, adding that casualty details were unconfirmed and the cause of the shooting was 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 Defence, 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 were slow to emerge, defence 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 "insider attacks" stem from personal grudges and cultural misunderstandings rather than Taliban insurgent plots.
Incidents in which Afghan troops 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 so-called "guardian angels".
In London, the British ministry of defence issued a statement saying: "We are aware of reports of an incident at Qargha. The incident is under investigation and it would be inappropriate to comment further at this time."
The Afghan National Army Officer Academy was hit by another "insider attack" last year when an Afghan soldier shot and injured 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.