A second official expected the drill could be wrapped up this week.
Iran has been furious over US President Donald Trump’s decision to pull out of an international nuclear deal and reimpose sanctions on Tehran.
Senior Iranian officials have warned the country would not easily yield to a renewed U.S. campaign to strangle Iran’s vital oil exports.
In addition, Mr Trump and Iranian President Hassan Rouhani have traded verbal blows in recent weeks, while the rocket attacks – perpetuated by the Iran-aligned Houthis – has raised the temperature still further.
The US military’s Central Command yesterday confirmed it has seen an increase in Iranian naval activity, including in the Strait of Hormuz, a strategic waterway for oil shipments that Iran’s Revolutionary Guards have threatened to block.
Navy Captain Bill Urban, the chief spokesman at Central Command, which oversees US forces in the Middle East, said: “We are monitoring it closely, and will continue to work with our partners to ensure freedom of navigation and free flow of commerce in international waterways.”
Central Command did not update its guidance on Thursday.
A third official said the Iranian naval operations did not appear to be affecting commercial maritime activity.
Anonymous US officials said the drills appeared designed to send a message to Washington, which is intensifying its economic and diplomatic pressure on Tehran but so far stopping short of using the US military to counter Iran and its proxies.
But Iran did not appear interested in drawing attention to them.
Iranian authorities have yet to comment on them and several officials contacted by Reuters declined to comment.
Trump’s policies are already putting significant pressure on the Iranian economy, although US intelligence suggests they may ultimately rally Iranians against the United States and strengthen Iran’s hardline rulers, officials say.
Iranian leaders are also believed to be angry at suggestions that the Trump administration is encouraging further demonstrations within Iran in the hope of toppling the ruling theocracy in the Islamic republic.
The country’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.
Protests have broken out in Iran since the beginning of the year over high prices, water shortage, power cuts and alleged corruption.
On Tuesday, hundreds of people rallied in cities including Isfahan, Karaj, Shiraz and Ahvaz to protest high inflation caused in part by the weak rial.