Just one month ago, President Donald Trump's top envoy for Iran told a major energy conference that oil market conditions are making it easier to choke off the Islamic Republic's crude exports without causing a price spike.

"When you have a better supplied oil market, it allows us to accelerate our path to zero," U.S. Special Representative for Iran Brian Hook said at CERAWeek by IHS Markit in Houston.

Since then, the cost of U.S. crude oil has jumped 12%, international Brent crude is trading above $70 a barrel, and the national average gasoline price is up 30 cents a gallon.

The primary reason for the run-up is simple: The market is tightening. That means a global oversupply of crude is draining, bringing supply and demand into balance and putting the market at risk of flipping into shortage.

On Thursday, the International Energy Agency said global oil supplies are tightening in the second quarter as OPEC and its allies slash production and U.S. sanctions on Iran and Venezuela look increasingly effective.

"Tightness in the oil market, however, is not just a supply story. In recent months, the resilience of demand has received less attention than the vicissitudes of production, but it is very important too," said IEA, a Paris-based energy policy adviser.

That tightening will make it more difficult for Trump to justify significantly curtailing Iran's crude shipments next month. In just a few weeks, the president must decide whether to extend waivers that allow several countries to import oil from Iran, which is under wide-ranging U.S. economic sanctions.

To be sure, analysts doubt Trump will refuse to extend the waivers, despite administration officials repeatedly invoking the administration's goal of driving Iran's oil exports to zero.