"We are still working with (Saudi) authorities to try to find him. We assume he is kidnapped, but we don't have any more information," US embassy spokesman Robert Keith told AFP.

Statements posted on Islamist websites in the name of the terror network of Saudi-born Osama bin Laden, which wants to "cleanse" the country of "infidels," took responsibility for Saturday's murder and the first reported case of kidnapping in the Saudi capital.

"Our fighters of the Fallujah Brigade in the Arabian peninsula have kidnapped an American, a Christian, Paul M. Johnson Jr. born in 1955 and working as an aeronautics engineer," said the statement signed "al-Qaeda in the Arabian peninsula."

Several photographs of the American, a copy of his work permit, driving licence and health card, according to which he was working for top US defence contractor Lockheed Martin and came from Maryland, were included.

But telephone numbers listed with the claim showed Johnson worked, possibly on a contract basis, with Advanced Electronics Company (AEC), the same Saudi firm in which the American killed here on Saturday was employed.

This was confirmed by sources in Riyadh, who named the US national killed in a drive-by shooting as Kenneth Scroggs. He was in his mid-fifties.

Company officials could not be immediately reached for comment, but a Saudi who knew both Americans described them as "extremely polite".

"They're super polite, better than the Arabs. This is a deviant, reckless minority attacking Americans," he said, requesting anonymity.