Danish authorities have ordered a ban on 27 ships linked to Russia from calling at their ports and receiving bunkering or supply services.
The restrictions follow the introduction of the 14th package of sanctions by the European Union against Russia.
Pilots have been warned that vessels in the Russian shadow fleet may attempt to change their names, making verification of their IMO numbers essential.
According to reports from the specialized website Maritime Bell, many of these ships linked to Russia used to call at the port in Skagen to stock up on supplies and fuel oil before continuing their voyage.
Russia transports about a third of its oil exports by sea through the Danish Straits, and about a third of these vessels are apparently without a recognized form of insurance.
Translation by Giles Foster