Security vendors are warning that two U.S. Department of State Web sites based in Russia could contain malware and should be avoided.

The most serious compromise was on the Web site for the U.S. Consulate General for St. Petersburg. About a week ago, researchers at Sophos PLC discovered that the site had been hacked and was apparently serving up malicious software to visitors.

The compromise seems to have been short-lived. By the time researchers were able to check the site manually, the infection had been cleaned up. By looking at a cached version of the page in question, however, they were able to find the malicious code in question.

A State Department spokeswoman said she was unaware of any breach.

As of Thursday, Sophos customers were still being blocked from accessing the St. Petersburg consulate Web server, which is hosted on the stpetersburg.usconsulate.gov and www.stpetersburg-usconsulate.ru domains.

The St. Petersburg consulate site was probably not deliberately targeted because it was one of about 400 sites infected by the criminals behind the hack, said Ron O'Brien, a senior security analyst with Sophos." The malware writer was looking for vulnerable sites and happened upon that site," he said.

Attackers were using these compromised servers to install Trojan software on victim's computers, O'Brien said. "It was a malware that allowed for remote access and it also attempted to download additional malware from a remote server."

These type of Web-based attacks have become increasingly common over the past years as criminals have created a number of kits, designed to lure victims to compromised Web sites and where unauthorized software is installed on their PCs. Webattacker is the best-known of these kits.

Separately, McAfee's SiteAdvisor software is now warning Web surfers not to visit the State Department's Moscow embassy Web site. According to a SiteAdvisor alert, this site has been associated with e-mail messages that contained computer viruses.

"After entering our e-mail address on this site ... we received two e-mails that contained a virus," McAfee said in the alert.