A malicious computer worm has been found that targets Apple Computer Inc.'s Mac OS X operating system, believed to be the first such virus aimed specifically at the Mac platform. The worm is called OSX/Leap-A, according to a posting on the Web site of antivirus software company Sophos, which said the worm is spread via instant messaging programs.

The worm attempts to spread via Apple's iChat instant messaging program, which is compatible with America Online's popular AIM instant messaging program, according to the Sophos Web site.

The worm sends itself to available contacts on the infected users' buddy list in a file called "latestpics.tgz," according to the Sophos Web site.

"This first Macintosh OS X threat is an example of the continuing spread of malicious code on to other platforms," said Vincent Weafer, senior director at Symantec Security Response, in a statement.

The worm will not automatically infect Mac computers, but will ask users to accept the file, Weafer said.

Symantec ranked the new worm as a Level 1 threat (with 5 being the most severe).