The United States is responsible for nearly half of the world's spam email, claims security firm Symantec.

In a study of spam received over the second half of 2006, Symantec found that roughly 45 per cent of the messages were sent from computers within the US. The volume of spam coming from the US was more than seven times as much as the second-largest offender, China.

"The purveyors of illicit pharmaceuticals, gurus of pink sheet penny stocks, and so-called representatives of 'your bank' may very well be from China, Russia, Brazil, and other countries, but the spam itself is sent mostly through American computers," according to Symantec.

The most likely cause of the high rate of stateside spam is because spammers prefer to use American internet service providers and free e-mail services (such as Hotmail or Yahoo Mail) to spread junk mail.

The news comes as the problem of junk mail continues to plague internet users, thanks to spammers increasingly using what is called image-based spam. Spammers use images for a variety of reasons but mainly to avoid detection.

This spam is able to bypass spam filters because they contain only a minimum of text. This means there's little for keyword-based filters to use to identify email as spam.

The first junk emails that used images were just graphics attached to an email, but this was soon followed by embedded coloured backgrounds. This also means hackers can hide malware such as Trojan Horses in the emails more easily.

As well as bypassing filters, the size of these emails is also putting a burden on servers. The average size of a spam message has increased by 77 per cent since September last year and continues to grow, according to another security company, SoftScan.