Western sanctions imposed on Russia for invading Ukraine and aimed at cutting its oil revenues have led to the rise of a vast "shadow fleet" of tankers helping Moscow keep its crude exports flowing.

The UK will sanction as many as 100 oil tankers that it says are part of a shadow fleet of vessels helping Russia to move its oil.

Estonia warned on Thursday that Russia appears willing to defend with military force its shadow fleet of aging tankers dodging international sanctions to keep oil revenue and equipment flowing, even through the territorial waters of European countries.