The final report has been submitted to the UN Security Council, and is likely to be published later this week, reports the BBC.
The firms are sister companies and share the same director. They have denied any wrongdoing.
The UN report has also claimed that between 2011 and 2014 "transactions valued at more than $2 million - alleged proceeds from the sale of goods in North Korea - flowed from an account that OCN and T Specialist set up in a North Korean bank, Daedong Credit Bank, to T Specialist's bank accounts in Singapore, the BBC reported.
The companies' lawyer, Edmond Pereira, has confirmed they are under investigation by Singapore authorities, but insisted they did not have any current financial links, interests, or any sort of relationship with entities in North Korea.
Pereira acknowledged that his clients "have done business with North Korean entities... before the UN sanctions came into force".
In November 2017, Singapore banned trade with North Korea entirely. Before that, some trade was allowed.
Global sanctions against North Korea have tightened considerably over the last two years as Pyongyang has continued to conduct nuclear tests and launch missiles.
(This story has not been edited by Business Standard staff and is auto-generated from a syndicated feed.)