Up to 20 North Korean bank accounts are suspected of being involved in illegal transactions, including counterfeiting and money laundering.
That is according to a South Korean newspaper, which says South Korea informed the U.S. about the accounts in China and Switzerland at Washington's request.
Pyongyang officials say the accusations are not true.
The U.S. is already considering financial sanctions against the north separate from proposed UN sanctions for its second nuclear test.
Meanwhile, President Obama's special envoy on North Korea ruled out any military action against the belligerent regime.
In the past month, North Korea has tested a nuclear weapon, test-fired several short-range missiles, and imprisoned two American journalists.