Investigations by the Internet Watch Foundation (IWF) found nearly 2,500 US sites containing illegal images.
The IWF study also said that some sites that contain the illegal content remain accessible for up to five years despite being reported to relevant authorities.
In April the US Attorney General proposed changes in the law to tackle the problem.
The proposals by Alberto Gonzales included new laws that would require Internet Service Providers (ISPs) to report images of child abuse.
In June a Technology Coalition comprising Microsoft, Time Warner AOL, Yahoo, Earthlink United Online and the National Center for Missing and Exploited Children (NCMEC) also announced plans to tackle online child abuse in the US.
The IWF, which is based in the UK, said that the reason why the US hosted the majority of illegal content was because the country has the most ISPs and the most web traffic in the world.
The new figures from the IWF are a snapshot of online child abuse content around the world.
The foundation relies on web users reporting illegal content to its website or hotline. Other countries including the US run similar services.
In the first six months of this year the IWF received more than 14,000 reports of suspected websites, a 24% increase on the first six months of 2005.
"The increase in reports processed could be attributed to a number of factors, including public intolerance of child abuse content online combined with increased awareness of our role in combating it," said Peter Robbins, chief executive of the IWF.
Of the reports, nearly 5,000 contained images of child abuse.
Of these nearly 2,500 were traced to the US and more than 730 to Russia.
One site was first reported to the IWF in 1999. Since then it has been reported to the foundation a further 96 times.
The IWF said that it had reported the site to the authorities in the countries where the website was hosted on 20 separate occasions.
However, the website "hopped" between the US and Russia every couple of days making it difficult to trace and shut down.
As a result it was still accessible to offenders, the IWF said.
A further 8% of 287 websites contained child abuse images remained accessible for between one to five years despite being reported by the IWF to relevant authorities.
The report also highlights the worst offending countries for hosting commercial and non-commercial child abuse content.
Non-commercial content is shared between offenders using tools such as free online photo albums or message boards to distribute pictures.
UK Sites reported to the IWF are shut down within 48 hours
The US was found to host 57.7% of commercial images of child abuse and 49.5% of non-commercial.
Russia hosts a further 28.1% of commercial content, and Japan 14.6% of non-commercial.
Other countries that feature in the list include Spain, Thailand and South Korea.
The report said that the UK did not host any commercial sites containing images of child abuse and was responsible for 0.2% of non-commercial sites.
Any sites reported to the IWF that are hosted in the UK are removed within 48 hours by UK ISPs.
"That only 0.2% of child abuse content is hosted here is a testament to the public's help in reporting suspicious websites and to all our partners," said Mr Robbins.
The IWF is funded by the EU and UK internet industry, including ISPs, mobile operators, internet search providers and telecommunications and software companies.
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