Suspicions are growing that North Korea has resumed forging $100 dollar bills that are so realistic that they are virtually indistinguishable from genuine currency.

It took a team of forgery specialists at South Korea's KEB Hana Bank to confirm that a single $100 bill found at a branch in Seoul in November was a fake. The discovery has triggered alarm because authorities have no idea how many similar "supernotes" - named because they are so similar to real banknotes - are now in circulation.

While previous "supernotes" were dated either 2001 or 2003, the new forgeries are dated 2006 and are even more sophisticated in the ink, the printing processes and the material that they use.

The forgeries also change colour when they are viewed from a different angle, just as genuine notes should, while the slightly rough texture of the note has also been accurately copied, a spokesman for Hana Bank told South Korea's Chosun Ilbo newspaper.

The 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.

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