The pressure has been on Iran's customers since May, to cut their purchases of Iranian oil and freeze out the oil producer, OPEC's third largest, from the world oil market. The U.S. is re-imposing sanctions on Iran following the U.S. withdrawal from the 2015 nuclear deal, and has threatened secondary sanctions on any country that continues to buy oil from Iran.

The campaign has proven effective thus far. In the first half of August, Iran's oil exports fell by 600,000 bpd, plunging from 2.32 million bpd to 1.68 million bpd. Iran's exports have been falling all year, and reached their lowest level in four months by July, before taking a real plunge in August.

Major customers, including South Korea, have suspended imports. China, despite some defiant posturing in the face of U.S. challenges, scaled back its purchases. While China has given no sign that it will comply with the U.S. directive, reports indicate that it is willing to at least halt any increase in Iranian purchases after sanctions are re-imposed on November 4.