Shadow fleet tankers hauling sanctioned oil cargoes have been involved in at least 50 incidents including collisions and oil spills, according to insurer Allianz.

Steadily increasing sanctions placed on Russia and existing regimes targeting Venezuela, Iran and North Korea have led to a rise in a fleet of poorly inspected and elderly vessels often with inadequate insurance, it said in a preview to a major report on safety and shipping to be published next week.