Two US advisors were shot dead in the Afghan interior ministry by an Afghan colleague after mocking anti-American protests over the burning of the Koran, a government source said Sunday.

"The advisors were scolding the protesters and calling them bad names. They called the Koran a bad book in the presence of the guy," the source said, requesting anonymity because of the sensitivity of the issue.

"After all this the guy had verbal arguments with the advisors and was threatened by them. He gets angry and shoots them. Eight rounds were fired at them.

"He then sneaks out and disappears. No one knew about the incident for more than an hour because the room is soundproofed," he said, adding that CCTV cameras had been viewed in the investigation of the shooting.

Asked about this description of events, a spokesman for the US-led International Security Assistance Force said: "The investigation is ongoing."

Government sources said police were hunting for an Afghan intelligence official suspected of killing the two Americans.

The interior ministry confirmed that "the suspect is one of the employees of the ministry and he is at large".

Local television quoted a source which named the suspect as 25-year-old Abdul Saboor, who had studied in Pakistan and joined the ministry as a driver in 2007 before being promoted.

He had signed into the ministry on Saturday before disappearing. The two US officers were found dead in their office with gunshot wounds. Saboor's family has been questioned, police said.

NATO on Saturday pulled all its staff out of Afghan government ministries after the shooting, which came as anti-US protests raged over the burning of Korans at a US-run military base.

Taliban insurgents have claimed responsibility for the shooting, saying it was in revenge for the Koran burning -- an incident that forced US President Barack Obama to apologise to the Afghan people.

NATO, which has a 130,000-strong US-led military force fighting an insurgency in Afghanistan, has advisors throughout the Afghan government but commanding officer General John Allen ordered them all withdrawn after the shooting.