The United States will "very soon" announce a new list of countries that will receive waivers to financial sanctions on oil trade with Iran, a U.S. official said on Thursday.
The latest round of U.S. sanctions come into effect on June 28 and aim to cut Iran's oil revenue to pressure Tehran into halting its nuclear program, which Western powers suspect is aimed at developing arms.
The United States can exempt countries from sanctions if they make significant reductions to crude imports from Iran, and granted a waiver to Japan and 10 European countries in March.
The official, who is familiar with the talks on sanctions and asked not to be identified, declined to give more details on which countries would join the exemptions list, but said the United States and South Korea had made progress in talks.
Even if countries are omitted from the list, it does not necessarily follow that the United States would quickly impose sanctions after June 28, the official said.
It would take some time for the U.S. to gather evidence to support punitive measures against financial institutions that have processed oil transactions, the official added.
Around two thirds of Iran's crude exports flow to Asia, where the biggest buyers are China, Japan, India and South Korea.
The four countries have already cut their imports by about a fifth from the 1.45 million barrels per day (bpd) they were buying a year ago as they prepare for the U.S. sanctions to come into effect.
Iran's Asian importers are also struggling to find ways around European Union sanctions that prevent EU insurers from covering shipments of Iranian crude.
Europe's dominates the world's tanker insurance market and without its insurers, Iran's crude buyers in Asia may be forced into more drastic import cuts.
Washington has encouraged oil producers, including ally and top exporter Saudi Arabia, to pump more to ensure there is enough crude in the market to cushion the impact of the sanctions.
Plentiful supply and faltering global growth have pushed international Brent crude prices down to around $100 a barrel from a March peak of $128, making it less costly for buyers to find alternatives.