Seoul's Ministry of National Defense said its troops had been put on alert against the e-mails, although no military information was thought to have been compromised.

The alert, issued last week by South Korea's Defense Security Command, the nation's counterintelligence agency, warned all personnel not to keep official data on personal computers and to keep all antivirus and other security software up to date.

Defense officials told reporters the warning followed successful phishing attacks against some military personnel using e-mails headed "Current state of the North Korean army's capabilities."

As is standard with phishing e-mails, opening an attachment installed hacker programs on the infected computer that allowed data to be stolen.

One report, carried by the Chosun Ilbo newspaper, said "hackers believed to be operating from China" were thought by authorities to be behind the attacks, using the e-mail addresses of South Korean service-members harvested from Web sites such as those of military units or veterans associations.