$5.56 billion stuck in bank accounts, with a similar amount held up in Japan since the beginning of the year, according to sources.
In China, Kunlun Bank, owned by China National Petroleum Corp, holds the Iranian oil dues. Iran has previously said China, its top oil buyer, owes it around $22 billion.
The 1 million bpd ceiling on Iranian exports is sharply below the pre-sanctions level of around 2.5 million bpd and gives Iran very little room to boost sales. Its oil exports in the first nine months of the year have averaged about 1.08 million bpd, according to Reuters data.
Of the total, China bought nearly half, followed by India and Japan, which bought about 194,000 bpd each. South Korea purchased 137,000 bpd, while Turkey took about 100,000 bpd.
HPCL's Namdeo said the refiner would resume crude imports if Iran continued to offer current conditions. He did not elaborate.
Industry sources have said Iran is offering free delivery, which saves on freight rates of up to $1 a barrel, and a small discount on the crude price.
India's shippers, meanwhile, would be keen to resume voyages to Iran if they can now get full insurance, the Shipping Corporation of India said in an email, adding it would ask New Delhi to use Indian vessels.
Until now, buyers of Iranian oil had to show a continuous reduction in purchases to qualify for a six-month waiver from US sanctions. The next review for most of the buyers is due soon.