WASHINGTON — The Trump administration on Wednesday imposed sanctions on an elaborate shipping network that Iran uses to sell oil, and unveiled a $15 million reward to anyone with information that disrupts the scheme, stepping up its effort to exert pressure on the Iranian economy.

The sanctions were the latest in a flurry of actions taken by the United States in recent days to further isolate Iran in hopes that it will return to the bargaining table to renegotiate an international agreement over its nuclear program. President Trump withdrew from the 2015 agreement in May 2018 and reinstated sanctions to limit Iranian oil sales, raising tensions with countries in Europe and Asia that have become reliant on Iran for energy.

The Treasury Department placed sanctions on 26 individuals and “entities” affiliated with the Islamic Revolutionary Guards Corps Quds Force, which the United States said has shipped approximately $500 million worth of Iranian oil in the last year. The sanctions freeze any assets held in the United States of those affiliated with the shipping network and prohibit them from doing business with Americans. The action also identifies 11 ships, placing anyone who owns or operates them on a Treasury list and exposing any port that lets them in, or firms that fuel or offload them, to future sanctions.

The shipping network is run by Rostam Ghasemi, Iran’s former minister of petroleum, and the United States said it has been used to facilitate the flow of money to Hezbollah and the government of President Bashar al-Assad of Syria. The United States considers Hezbollah a terrorist organization even though members of the group have embedded into legitimate parts of the Lebanese government.