North Korean ships have been trading coal and oil in violation of UN Security Council sanctions. 

That’s according to a report by the Wall Street Journal published Tuesday. 

It quoted a diplomatic source as saying that the UN and related authorities are investigating at least 40 ships and 130 companies over their alleged involvement in almost 200 cases of illegal ship-to-ship transfers of coal and refined oil.

The owners of the ships are not known, but they are reportedly registered with Taiwan and Togo. 

The daily said that North Korean ships are using an array of tactics to evade sanctions, including forging documents or switching off ships' tracking beacons.  

It added that five nations including the U.S., Japan and Australia are conducting air patrols in Asian waters to monitor the North's illegal ship-to-ship transfers.