"CIA Tango down," a member of Anonymous said on @YourAnonNews, a Twitter feed used by the group. "Tango down" is an expression used by the US Special Forces when they have eliminated an enemy.

Attempts to access the CIA website at cia.gov were unsuccessful.

A CIA spokesman had no immediate comment.

Anonymous last month briefly knocked the websites of the US Justice Department and the Federal Bureau of Investigation offline.

Those attacks were in retaliation for the US shutdown of file-sharing site Mega upload.

There was no immediate explanation from Anonymous for the targeting of the CIA site.

Most Anonymous cyber attacks are distributed denial of service attacks in which a large number of computers are commanded to simultaneously visit a website, overwhelming its servers.

Copyright AFP (Agence France-Presse), 2012