It's believed that North Korea may have developed a way to create extremely good replicas of American $100 bills.

A number of fakes notes have turned up in neighboring South Korea whose quality is so high that they are virtually indistinguishable from the real thing.

It ended up taking a team of forgery specialists at the country's KEB Hana Bank to confirm that a single $100 bill found at a branch in Seoul in November was a fake.

It's believed North Korea is forging $100 dollar bills. The notes are so realistic they are virtually indistinguishable from genuine bills

The worry is now that there may be many more 'supernotes' that are now in circulation, but authorities simply have no idea.

Other fakes were dated either 2001 or 2003, but the new forgeries are dated 2006 and use even better techniques with more sophisticated ink, improved printing and paper.

Even features that are designed to stop fakes from being created have been incorporated into the forgeries that change colour when viewed from a different angle, just as genuine notes should.

The high quality of the forgeries has immediately led to suspicion falling on North Korea, which has a track record of forging foreign banknotes in order to earn hard currency for the regime.

Authorities are now concerned there may be many thousands now in circulation

In order to print supernote-level forgeries, a minting corporation-level production line would need to be in place which itself would cost tens of millions of dollars.

Pyongyang is in need of funds as international sanctions imposed due to the regime's ongoing development of nuclear weapons and long-range ballistic missiles begin to take hold.

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'It seems that whoever printed these supernotes has the facilities and high level of technology matching that of a government', Lee Ho-jung, a bank spokesman said The Daily Telegraph. 'They are made with special ink that changes color depending on the angle, patterned paper and Intaglio printing that gives texture to the surface of a note'.

Fake $100 bills are nothing new with earlier versions having been around for at least 15 years.

US government officials estimated in 2006 that as much as $250 million worth of fake $100 bills could be in circulation worldwide.

The North Korean regime would have the bills distributed overseas via the use of criminal groups and would also be delivered by transporting them through the diplomatic pouch to embassies from where they could be slipped into circulation.