The Pentagon has approved a major expansion of its cybersecurity force over the next several years, more than quadrupling its size to bolster the nation's ability to defend critical computer systems and conduct offensive computer operations against foreign adversaries, according to U.S. officials.

The move, requested by the head of the Defense Department's Cyber Command, is part of an effort to build an organization that until now has focused largely on defensive measures into the equivalent of an Internet-era fighting force. The command, made up of about 900 personnel, will expand to include 4,900 troops and civilians.

Details of the plan have not been finalized, but the decision to expand the Cyber Command was made by senior Pentagon officials late last year in recognition of a growing threat in cyberspace, said officials, who spoke on condition of anonymity because the expansion has not been formally announced. The gravity of that threat, they said, has been highlighted by a string of sabotage attacks, including one in which a virus was used to wipe data from more than 30,000 computers at a Saudi Arabian state oil company last summer.