The US and China host nearly two-thirds of spam, viruses and other computer security threats delivered around the world in 2006, an AFP report said.
The AFP report quoted computer security firm Sophos as saying that 34.2% of the so-called malware last year originated from the US, with 31% from China.
Russia was third, accounting for 9.5 % of the threats, the AFP report said.
The report added that Sophos identified 207,684 different threats, ranging from spam, viruses and 'trojans' that download programs to infect computers, to 'ransomware' designed to 'kidnap' data by encrypting it, and provide the password once a ransom has been paid.
The report said 90% of all spam is now relayed from 'zombie computers' infected with some kind of malicious code.
Sophos said its 'dirty dozen' list of worst spam-relaying nations was again headed by the US, accounting 22% of the spam sent worldwide.
China was second with 15.9%, followed by South Korea (7.4%), France (5.4%), Spain (5.1%), Poland (4.5%), Brazil (3.5%), Italy (3.2%), Germany (3%), Britain (1.9%) and Russia and Taiwan (each with 1.8%).
Sophos noted however that because spam is mostly relayed from zombie computers, the relaying PC does not need to be based in the same country as the computers being used to send the spam, the AFP report said.