The Trump administration is poised to tell five nations, including allies Japan, South Korea and Turkey, that they will no longer be exempt from US sanctions if they continue to import oil from Iran.
US Secretary of State Mike Pompeo plans to announce on Monday that the administration will not renew sanctions waivers for the five countries when they expire on May 2. The other two are China and India.
It was not immediately clear if any of the five would be given additional time to wind down their purchases or if they would be subject to US sanctions on May 3 if they did not stop.
The decision not to extend the waivers was finalised on Friday by US President Donald Trump.
Officials said it was intended to further increase pressure on Iran by strangling the revenue it gets from oil exports.
The administration granted eight oil sanctions waivers when it reimposed sanctions on Iran after Mr Trump pulled the US out of the 2015 nuclear deal.
They were granted in part to give those countries more time to find alternative energy sources and to prevent a shock to global oil markets from the sudden removal of Iranian crude.
US officials say they do not expect any significant reduction in the supply of oil given production increases by other countries, including the US.
Since November, Italy, Greece and Taiwan have stopped importing oil from Iran. The other five have not, and have lobbied for their waivers to be extended.
Nato ally Turkey has made perhaps the most public case for an extension, with senior officials telling the US that Iranian oil is critical to meeting their country’s energy needs.
They have also made the case that as a neighbour of Iran, Turkey cannot be expected to completely close its economy to Iranian goods.