Britain warns of Trojan horse computer attacks.

Computer experts are warning of an increase in targeted computer attacks worldwide.

In such attacks, virus writers craft Trojan horse programs to sneak into computers and steal documents. Recently, Great Britain's central government computers were targeted by these widespread and sophisticated attacks.

Central government computers have been the most popular target, but corporations and individuals are also at risk, according to a warning from the British National Infrastructure Security Coordination Center (NISCC).

"The attackers' aim appears to be covert gathering and transmitting of commercially or economically valuable information," the warning stated. The attacks had recently become more sophisticated, according to the agency.

The warning did not mention specific agencies or firms that had been attacked. But the warning listed more than a dozen Trojan horse programs that the agency said had been used in the attacks since January.

In many cases, employees are tricked into installing the malicious programs by clever e-mails loaded with infected documents. Sometimes, the attackers download publicly available documents off the Internet, load them with the Trojan horse, and then e-mail them to carefully selected employees who would be likely to open such a file. To make the notes even more realistic, the e-mail appears to come from a co-worker.

"The attackers are able to receive, Trojanise, and resend a document within 120 minutes of its release, indicating a high level of sophistication," the warning said. The attacks normally focused on individuals who have jobs working with commercially or economically sensitive data, according to the NISCC.

The warning also said anti-virus software and firewalls do not give complete protection, and "there is no complete mitigation for computers connected to the Internet."

An NISCC spokesman could not tell the media how many computers were attacked or provide additional details. An anonymous spokesman told the Associated Press that the agency is "not aware of any loss of information."

Experts say such targeted attacks are on the rise. Howard Schmidt, former White House cybersecurity advisor, told silicon.com that it was unlikely that the virus writers have limited their attack to U.K. institutions, suggesting U.S. firms and agencies might also be at risk.