US sanctions on Iran and Russia are warding off buyers and stranding vessels laden with their oil at sea, Reuters reported on Wednesday citing trade sources and analysts.

A series of stepped-up US sanctions beginning in October on Russian and Iranian tankers, companies and entities facilitating their oil trade is increasingly hampering oil exports which are the main source of revenue for both countries.

US President Donald Trump this month reinstated the so-called "maximum pressure" campaign on Iran from his first term, aiming to drive its oil sales to zero.

Since the sanctions were ramped up in the fourth quarter of last year, OPEC member Iran has faced a challenge chartering tankers to move its supplies, Reuters cited Xu Muyu, a senior analyst at energy consultancy Kpler, as saying.

Kpler figures indicated that Iranian oil in so-called floating storage stood at an over one-year high exceeding 25 million barrels, with around 80% of the volume at sea off Malaysia and Singapore.

The total amount of Iranian oil at sea rose by as much as 20 million barrels already just since the start of 2025, three analysts cited by Reuters said.

A ban last month by the Shandong Port Group on US-sanctioned tankers has deprived Iran access to the operator of top crude importer China's largest oil terminals receiving Iranian, Russian and Venezuelan oil.

China accounts for 95% of Iran's oil exports, but it does not purchase the oil directly.

Instead, small independent refineries typically Iranian oil after it is blended with crude from other countries, ensuring it is not labeled as Iranian by Chinese customs in order to comply with sanctions against Iran.