Iran on Wednesday said a new naval destroyer was in the testing phase ahead of a deployment to the Strait of Hormuz.
In addition, the possible size of the exercise - 100 boats, mostly fast-moving vessels, and hundreds of Iranian troops - is raising USA concerns, as well as the timing of the effort, which normally comes later in the year.
A second official expected the drill could be wrapped up this week.
Iranian threats to close the Strait of Hormuz, a critical passageway for oil and natural gas shipments, have further heightened tensions with the United States, increasing the possibility for military escalation.
The exercise comes as rhetoric from the IRGC towards the United States has accelerated in recent days.
"We are aware of the increase in Iranian naval operations within the Arabian Gulf, Strait of Hormuz and Gulf of Oman", said Navy Captain Bill Urban, the chief spokesman at Central Command, which oversees U.S. forces in the Middle East.
US economic pressure on Iran has added to difficulties that the Tehran government has faced in getting the economy back on its feet and has sparked a series of protests.
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Iran's Islamic Revolutionary Guard is expected to put 100 gun boats in the Persian Gulf sometime in the next 48 hours to practice "swarm" tactics simulating actions which could potentially shut down the Strait of Hormuz, where roughly 10 percent of the world's oil passes through each year, two USA officials said Wednesday.
They said the drills could begin within the next 48 hours, although the precise timing was unclear.
They have also expressed concern about keeping open the waterways off Yemen, where rebels backed by Iran have attacked oil tankers. Iran's currency plumbed new depths this week ahead of August 7, when Washington is due to reimpose a first lot of sanctions following Trump's withdrawal from the 2015 nuclear deal.
In reply, Iranian Foreign Minister Mohammad Javad Zarif warned Trump to "be cautious", saying that they have been around for millennia and have seen the fall of several empires, including their own, which lasted more than the life of some countries.
On Tuesday, hundreds of people rallied in cities including Isfahan, Karaj, Shiraz and Ahvaz in protest against high inflation caused in part by the weak rial.
A number of protests have broken out in Iran since the beginning of the year over high prices, water shortage, power cuts and alleged corruption.