The Afghan had been working at an annex of the U.S. embassy when he turned on his colleagues on Sunday, killing the American citizen and injuring another, ABC reports.
U.S. embassy officials say the motivation for the shooting - in which the attacker was also killed - is not yet known.
The incident took place inside the Ariana Hotel compound, an annex to the U.S. embassy, which is believed to the Afghan headquarters for the Central Intelligence Agency (CIA).
"There was a shooting incident at an annex of the U.S. embassy in Kabul involving an Afghan employee who was killed. One U.S. citizen was killed, one was wounded," said US embassy spokesman Gavin Sundwall.
An Afghan government official speaking anonymously to AFP said the Ariana compound was used by the CIA.
The CIA has refused to comment, it reports.
It is the second major security breach in two weeks, Reuters reports, after a 20 hour-siege of the embassy district in mid-September. A week later, the government's peace envoy, former President Burhanuddin Rabbani, was assassinated.
Washington has accused elements of the Pakistani state of supporting the Al Qaeda-linked Haqqani network, which it blames for the September 13 embassy attack, and tensions between the US and Pakistan have reached an unprecedented level.
Pakistan prime minister Yousuf Raza Gilani said at the weekend the US allegations would only benefit the militants, and that they "betray a confusion and policy disarray within the US establishment on the way forward in Afghanistan".
His comments came after top US military officer Admiral Mike Mullen directly accused Pakistan's intelligence service of supporting the network's attack on the embassy and a truck bombing on a NATO outpost.