The Trump administration has softened demands that countries like China and India end all imports of oil from Iran. But Mr. Trump has nevertheless sought to line up support against Tehran as the United States prepares to reimpose sanctions on its energy sector.

At present, Iran exports around two million barrels of oil a day, equivalent to about 2 percent of global supply. But the United States wants to drastically curb Tehran’s ability to sell its natural resources, eating into any additional supply from Saudi Arabia, Russia or elsewhere.

“The administration is posturing, to a certain extent, with this hard-line position,” said Andrew Keller, a former state department sanctions expert who is now a partner at the law firm Hogan Lovells.

Still, analysts have been increasing their estimates of how far Iranian crude exports are likely to fall, from a few hundred thousand barrels a day to around a million. A loss in the upper end of that range would wipe out much of the added supply that oil producers had promised in Vienna.

That looming outage is far from the only potential disruption weighing on the oil markets.

Venezuela, once a major producer, has nearly halved output, to less than 1.4 million barrels a day, as its economy has cratered and international sanctions have been imposed on the country. Further declines seem almost certain.