More than 58 million barrels of Iranian crude and condensate was on-the-water as of July 1, according to data from Vortexa and Bloomberg calculations. More than 90% has no clear destination. The vessels are either indicating "for orders" or Singapore as their next port of call, a sign they may conduct ship-to-ship transfers in the Malacca Strait.

A failure to quickly sell the crude would deprive Tehran of much-needed revenue and could weaken its hand in negotiations with Washington. The Islamic Republic has until mid-August to find buyers after the US lifted sanctions on the oil in the middle of June and ended a blockade of Iranian ports, part of an interim peace deal.

Demand from Chinese independent refiners — Iran's main customers prior to the conflict — has been muted as the sector's run rates crash to a nine-year low. China's state-owned refiners have also stayed on the sidelines, citing concerns over the ability of banks to finance any deals.

Most of the oil is in and around the Persian Gulf, in the Indian Ocean or the Malacca Strait near Singapore.

Iran said on Wednesday that it had shipped more than 40 million barrels of oil since the US lifted its naval blockade. But more than 20 million barrels of Iranian crude has been idling in Asian waters for seven days or longer, up almost 18% from a week earlier, according to Kpler Ltd.

Tehran faces a number of obstacles in trying to sell the oil. European Union and UK restrictions are still in place, complicating insurance, while some ports may be hesitant to accept the dark-fleet vessels that Tehran uses to carry its crude. There's also a chance the barrels could get stranded mid-deal if President Donald Trump decided to end the window early.

Buyers remain wary that Washington could reimpose sanctions if negotiations collapse, US Treasury Secretary Scott Bessent told Fox News on Tuesday. "No one other than China, who was already buying it when it was sanctioned, has bought it, so it's still trading at a discount."

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The other major barrier to Iran offloading the crude is a lack of demand in major Asian markets, where there's been little interest despite Tehran's efforts to court buyers. The region is well-supplied, both with non-Iranian Persian Gulf oil that can now transit the Strait of Hormuz and crude from farther afield that was bought during the war.