An oil tanker runs aground off eastern China, leaking fuel into the water. Another is caught in a collision near Cuba. A third is seized in Spain for drifting out of control.

These vessels were part of a "shadow" fleet of tankers carrying oil last year from countries hit by Western sanctions, according to ship tracking and accident data and interviews with more than a dozen industry specialists.

Hundreds of extra ships have joined this opaque parallel trade over the past few years as a result of rising Iranian oil exports as well as restrictions imposed on Russian energy sales over the war in Ukraine, said the industry players, who include commodity traders, shipping companies, insurers and regulators.