Japan this week became the first of Iran's oil buyers to make a payment for crude imports under an interim nuclear deal sealed by Iran and the six world powers in Geneva in November.
Tokyo's role in sending the first funds may be a boon for Japanese firms jostling for position with international rivals to invest in Iran's oil and gas sector, Reuters said.
It is unclear why Japan was the first of Iran's oil buyers to pay. China, India and South Korea also buy crude from Iran and all have billions of dollars of cash held in Iranian accounts pending transfer.
The Iranian funds were released earlier this week from an account held by the Bank of Japan, three sources told Reuters speaking on condition of anonymity because of the sensitivity of the issue. One of the sources confirmed the amount was for $550 million, while another said it was likely further releases of Iranian funds would be made by Japan as they come due.
A substantial portion of frozen Iranian funds are held at the Bank of Japan, one of the sources said.
The funds were transferred to an Iranian Central Bank account in Switzerland, a US Treasury Spokeswoman said earlier this week.