Colombia and North Korea are the largest producers of fake US banknotes, a report suggests.
The study by the US Treasury Department, Federal Reserve and Secret Service said that one in every 10,000 greenbacks was a fake.
It said more affordable equipment meant counterfeiting was getting easier.
In Bulgaria, previously one of the biggest producers of fake dollars, fraudsters have turned to copying the euro, said the study.
There has been a growth in digitally printed notes - mainly in the US - although the authorities predicted this would become more common elsewhere.
'Highly deceptive'
About $450bn of the $750bn of US currency in circulation is held outside the US, with up to $70m estimated to be fake.
Colombia, the biggest supplier of illegal drugs in the US, was also the number one source of counterfeit money.
It accounted for about 15% of the 56.2 million counterfeit greenbacks in circulation in 2005.
North Korea is known among officials for its so-called supernotes.
"The US Secret Service has determined through investigative and forensic analysis that these highly deceptive counterfeit notes are linked to the Democratic People's Republic of North Korea and are produced and distributed with the full consent and control of the North Korean government," the report said.
In February this year, South Korea's intelligence agency said that North Korea was no longer forging US dollars.
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