Japan has been seeking a waiver that would allow it to continue importing Iranian oil, but it appears unlikely to win one, AFP reported.
US President Donald Trump in May pulled out of the 2015 nuclear deal between Tehran and last month began reimposing sanctions that block other countries from trading with Iran.
A second phase of sanctions targeting Iran's crucial oil industry and banking sector will be reinstated on November 5.
As a result, Japanese oil companies are preparing to halt imports of Iranian crude and researching ways to increase imports from elsewhere to make up the shortfall, the reports said.
A trade ministry official on Monday confirmed Japan had raised the issue of a waiver in talks with the US but declined to comment further.
Oil importers declined to confirm they were contingency planning for a halt in Iranian imports.
"We've been saying we will observe a government decision (on Iranian oil imports), but we can't comment further as we don't disclose information on individual trades," a spokesman for wholesaler Showa Shell Sekiyu said.
Wholesaler JXTG also declined to confirm the report.
Japan relies heavily on imports of oil from the Middle East, though crude from Iran accounted for just 5.3 percent of the country's total imports last year.