The US administration is "prepared to work with countries that are reducing their imports on a case-by-case basis", the official added.
"The US government's tough stance raised the stakes for a more significant Iran export loss than previously foreseen", said Norbert Ruecker, head of macro and commodity research at Swiss bank Julius Baer. This would signal an attempt to bypass USA sanctions on Iranian transactions using the dollar - the dominant currency in global oil trade.
Iran's crude exports fell further in the first week of October, according to tanker data and an industry source, taking a major hit from U.S. sanctions and throwing a challenge to other OPEC oil producers as they seek to cover the shortfall.
US Secretary of State Mike Pompeo said in India last month that the Trump administration would consider waivers for Iranian oil buyers such as India but they must eventually bring the imports to zero. Refiners in the country have placed orders to buy 9 million barrels from Iran in November.
The IMF said the Iranian economy was now expected to contract over the next two years "on account of reduced oil production, before returning to modest positive growth in 2020-23".
Citing the Kremlin, Reuters reported that Iranian Foreign Minister Mohammad Javad Zarif and his Russian counterpart Sergei Lavrov discussed ways to enhance Tehran-Moscow energy cooperation over the telephone on Monday.
In the United States, almost 40 percent of daily crude oil production was lost from offshore U.S. Gulf of Mexico wells on Tuesday because of platform evacuations and shut-ins ahead of Hurricane Michael.
That is the longest streak of weekly cuts since October past year.
"This time round, because there is so much less of a consensus on the sanctions from other countries, there will be even more of a push to obtain exceptions or find ways to walk around the sanctions".
President Trump made a decision to leave the deal and reimpose sanctions.
India has already stated that it will continue importing oil from Iran as they "do not recognise sanctions imposed by anyone other than the United Nations".
"Our goal remains to get to zero oil imports from Iran as quickly as possible, ideally by November 4", when the full spectrum of sanctions are re-imposed on Iran.
The best value zone for the November WTI crude oil market is $72.35 to $71.27.