The number of new viruses released on the Internet in May hit a 2-1/2-year high last month, an anti-virus vendor says.

Five new viruses released in May made Sophos' Top 10 for the month. Included are Sasser, Netsky-Z, Sober-G, Bagle-AA, and Lovgate-V, the company said Wednesday. Sasser led the pack in the number of infected machines reported.

Sophos found a total of 959 new viruses on the Internet in May, the highest number since December 2001. That number includes new viruses that were variants of older ones.

Also, security vendor Symantec Corp. on Wednesday upgraded the Korgo.F virus to a level 3 from a level 2, because of an increase in submissions, the company said.

"W32.Korgo.F includes backdoor functionality that could leave systems open to unauthorized access," Alfred Huger, senior director at Symantec Security Response, said in a statement. "This backdoor functionality could result in a loss of confidential data and may also compromise security settings."

Korgo.F attempts to propagate by exploiting a Windows XP and 2000 vulnerability reported by Microsoft on April 13.