Iran's Islamic Revolutionary Guard, a branch of Iran's Armed Forces, is touted to conduct the drills, which have the potential to shut down the Strait of Hormuz, through which almost ten per cent of the world's oil supplies are transported, two U.S. officials were quoted by Fox News as saying on Wednesday.
"We are aware of the increase in maritime operations in the Arabian Gulf, the Strait of Hormuz and the Gulf of Oman", spokesman for the Central Command, which oversees United States forces in the Middle East, Captain Bill Urban said.
A spokesman for the US Central Command said that the military was closely monitoring movements of Iranian troops in the area and would continue to work to guarantee freedom of navigation in the waterway.
In addition, the possible size of the exercise - 100 boats, mostly fast-moving vessels, and hundreds of Iranian troops - is raising US concerns, as well as the timing of the effort, which normally comes later in the year.
They said the drills, which typically happen much later in the year, could begin within the next 48 hours, although the precise timing was unclear.
USA media outlets say Iran has launched a large-scale military drill in the Persian Gulf.
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"Iran has threatened to close the Strait of Hormuz", Mattis said. Both countries have asserted the right for decades to block warships whose use of the waterway they don't consider to be "innocent passage", and Iran annually holds exercises in the strait.
The Iranian Revolutionary Guards Corps is a paramilitary force that answers directly to the Islamic republic's supreme leader.
Iranian authorities have yet to comment on them and several officials contacted by Reuters declined to comment.
Saudi Arabia announced last week that it was halting oil shipments through the Bab al-Mandeb strait that connects the Red Sea to the Sea of Oman after Houthi forces from Yemen attacked two Saudi-owned oil tankers passing through the waterway.
Rockford Weitz, who heads the Fletcher Maritime Studies Program at Tufts University, said that in the Strait of Hormuz, Iran "could damage commercial shipping with relatively cheap anti-ship missiles, fast patrol boats, submarines and mines".
"If they want to meet, I'll meet anytime they want, anytime they want".
It was the first time parliament had summoned the Iranian president, according to Reuters, which noted that he is under pressure from hard-line rivals to change his Cabinet after a deterioration in relations with the United States and Iran's growing economic difficulties.