Iran launched the exercise in the Gulf last week, sending dozens of small attack boats out into the Strait of Hormuz -- a vital, oil-shipping waterway that Iranian President Hassan Rouhani last month threatened to shut down.

"It's pretty clear to us that they were trying to use that exercise to send a message to us that as we approach the period of the sanctions here that they had some capabilities," US Central Command head General Joseph Votel told Pentagon reporters.

The capabilities include ocean mines, explosive boats, coastal defense missiles and radars, he said.

Votel said he saw Qassem Soleimani, who heads the external operations Quds Force for Iran's Revolutionary Guards, as being responsible for the exercise.

"He is an individual who is perpetrating a lot of this destabilizing activity," Votel said.

"Wherever you see Iranian activity, you see Qassem Soleimani."

The United States on Tuesday reimposed sanctions on Iran, after President Donald Trump pulled out of the 2015 multilateral nuclear deal with Tehran.