|Some armed groups aim to derail NATO's intention to transfer security operations to Afghan forces by 2014 [EPA]
An Afghan employee has killed a US citizen and wounded another in a "shooting incident" at an annexe of the US embassy in Kabul, which is reported to also serve as the Central Intelligence Agency's (CIA) base in Afghanistan.
"There was a shooting incident at an annexe of the US embassy in Kabul involving an Afghan employee who was killed," US embassy spokesman Gavin Sundwall said on Monday.
Sundwall said that the Afghan employee had acted as "a lone gunman" and that the motive for the attack was "still under investigation".
The injuries to the second US citizen were not life-threatening, Sundwall added.
The CIA has declined to comment, but a US government source acknowledged that the building targeted in Sunday's attack likely contained CIA offices.
Analysts say some groups in Afghanistan want to derail NATO's plans to handover security responsibility to Afghan forces by 2014.
Kabul, at one time considered a relatively secure area, has witnessed several recent attacks.
Anders Fogh Rasmussen, NATO's secretary-general, recently told reporters: "We are witnessing that the Taliban try to test transition but they can't stop it. Transition is on track and it will continue."
Last week, Burhanuddin Rabbani, Afghanistan's former president who headed a team tasked with negotiating peace with the Taliban, was killed in a suicide attack on his home.
Two weeks ago, fighters launched an assault against the US embassy and NATO headquarters in Kabul.
US officials blamed those attacks on the Taliban-linked Haqqani network, an Afghan group based in Pakistan's tribal areas.
The officials said there was intelligence, including intercepted phone calls, suggesting those attackers were in communication with people connected to Pakistan's principal spy agency, the Inter-Services Intelligence (ISI) directorate.
Pakistani officials strongly denied any ISI connection to the earlier Kabul attacks.