Iran oil shipments are declining at a faster-than-expected pace ahead of U.S. sanctions set to begin in November.

Iran expects crude exports to fall by a third in September, according to people familiar with purchasing plans, potentially posing an unforeseen supply risk to markets. Officials at the state-run National Iranian Oil Co. provisionally expect crude shipments to drop to about 1.5 million barrels a day next month, down from about 2.3 million barrels a day in June, say people familiar with the country’s ports loading...