One in six spam emails come from the US, according to a report from a security firm.
Sophos ranked the US as the top source of the unwanted messages around the world, sending 15.6 per cent of all spam globally for the second quarter of the year.
Sophos security researcher Graham Cluley called for the US to clean up its spam problem.
"Barack Obama's recent speech on cybersecurity emphasised the threat posed by overseas criminals and enemy states, but these figures prove that there is a significant problem in his own back yard," he said in a statement.
"If America could clean up its compromised PCs it would be a considerable benefit to everyone around the world who uses the net," he added.
However, the report had praise for Russia. This time last year, Russia was second to the US, sending out 7.5 per cent of global spam. This year, it's slipped to ninth position in the ranking, and sends just 3.2 per cent of the unwanted messages. See below for the full list.
New methods
Sophos also said that spammers are taking to social networking sites, such as Twitter, using URL shortening services such as TinyURL, bit.ly and others.
The 140 character limit on Twitter means many users turn to such services to shorten long URLs, but the shortened links obscure the actual target, letting spammers and malware writers send users to sites they might not otherwise visit.
"This is being exploited by hackers that will use the services to obscure links to offensive material or malicious websites, and then distribute the links in spam emails, as well as posting them on Twitter and other networks," Sophos said.
Earlier this month, Symantec said shortened URLs now make up 2.2 per cent of all spam.
The top 12 worst countries for spam distribution:
1. United States (15.6 per cent)
2. Brazil (11.1 per cent)
3. Turkey (5.2 per cent)
4. India (5.0 per cent)
5. South Korea (4.7 per cent)
6. Poland (4.2 per cent)
7. China (4.1 per cent)
8. Spain (3.4 per cent)
9. Russia (3.2 per cent)
10. Italy (2.8 per cent)
11. Argentina (2.5 per cent)
12. Vietnam (2.3 per cent)
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