London, Singapore: Iran has reduced the amount of crude stored at sea in the past three months but still has as much as 20 million barrels anchored offshore, shipping and industry sources said.

Tehran typically struggles to sell its crude during summer months, when refiners are looking for lighter crudes to make more gasoline. This year, Western sanctions aimed at stymying financial transactions that underpin trade have made its crude an even tougher sell. The volume of crude in floating storage has steadily declined from about 33 million barrels in late July, and is half its peak of more than 40 million barrels in June.

Wide-ranging international sanctions implemented this year have excluded crude oil sales, but Western-allied powers have increased political pressure on countries to trade less with Iran.