MANILA, Philippines -- Manufacturers of spam are writing simpler email to elude more sophisticated filtering software, according to computer giant IBM.

Russia is the biggest source of spam, accounting for about 11 percent of total spam detected worldwide, according to IBM's midyear X-Force security report.

But more important, IBM noted a "back to basics" trend for spammers, in response to more sophisticated anti-spam technologies. Web-based email services like Gmail and Yahoo have also improved anti-spam features.

"The complex spam of 2007 -- image-based spam, file attachment spam, etc. -- has almost disappeared and now spammers are using simple URL spam," the report said.

A few simple words and a URL apparently make it difficult for spam filters to detect these email. Approximately 90 percent of all spam is now URL spam, IBM said.

Meanwhile, IBM's report also noted that online gamers continue to become preferred targets for cyber criminals. The top four password-stealing Trojans or viruses detected since January were all aimed at gamers, IBM said.

These viruses aim to steal gamers' "virtual" assets in order to sell them for real money in online market places, IBM said.

The IBM Internet Security Systems X-Force organization is composed of security experts who research on security issues and monitors end-user threats.