The United States is still the superpower of spam, beating China and South Korea to reclaim the dubious honor in the latest survey of the origin of unwanted e-mail clogging the world's inboxes.

Security firm Sophos also found in the survey of the "dirty dozen" top spamming nations that 'zombie computers' — hijacked PCs often in different countries than the spammer — were now to blame for 60 percent of the world's spam on the "dark side" of the Internet.

Between April and September of this year, 26.4 percent of the world's spam emanated from the United States, compared to 41.5 percent for the same period the year before, according to Sophos.

South Korea and China held the next two spots, with their spam production rising fast. South Korea was 19.7 percent of the world's spam, compared to 11.6 percent the year before.

China was pegged at 15.7 percent, up more than six percent from the same time period in 2004.

France (3.4 percent), Brazil (2.6), Canada (2.5), Taiwan (2.2), Spain (2.2), Japan (2.0), Britain (1.5), Pakistan (1.4) and Germany (1.2) rounded out the "dirty dozen."

"The United States and Canada have significantly reduced their role in the problem," Sophos said in a statement.

"Sophos has seen a sharp drop in spam sent from North American computers due to a number of factors: jail sentences for spammers, tighter legislation and better system security."

In August, three people in the United States were indicted and a fourth pleaded guilty to criminal charges in the first case related to the transmission of obscene spam e-mails under a 2003 US anti-spam law, known as

Sophos warned action against spammers on their home turf meant they would simply look to hijack unprotected personal computers in other countries to relay spam, without owners ever knowing.

"There are fortunes to be made from the dark side of the Internet and spammers who are finding it harder to sell goods via bulk email are likely to turn to other criminal activities," said Graham Cluley, senior technology consultant with Sophos.

Authorities from the United States and 25 other countries in May launched a new initiative to crack down on spam sent through hijacked computers, or "zombies."

Survey results were distilled from analysis by Sophos of its global network of spam traps.