Iran swiftly vowed to retaliate for what it called a deadly attack on Sunday.
Forces with the Revolutionary Guard Corps were observed preparing to launch rockets with sea mines into the strait, according to Tim Hawkins, a spokesperson for the US Central Command.
The US military last week completed clearing sea mines from the strait's international shipping routes.
Semiofficial news outlets in Iran reported sounds of explosions near Larak island on the strait. The Guard in a statement carried by Iran's state broadcaster noted "the martyrdom and injury of several of our fighters and compatriots."
A return to open conflict would be dangerous for the region, where Iran has targeted US military bases and infrastructure in Gulf countries since the war began on February 28.
The fighting comes just days after the Trump administration said it would turn its focus to increasing economic pressure - rather than military action - to try to end the war.
The shifting strategy centres on threats to punish any country or entity that continues to conduct business with Tehran.
Sunday's attack was a "fatal mistake by the Trump regime during the economic war" and the enemy will pay the consequences militarily and economically, a Guard spokesperson, General Hossein Mohebi, said in a statement carried by Iranian media.
The US turn to using sanctions came as the administration weighs diminished munitions stockpiles after months of war, sparking concerns that the prolonged conflict could undermine US military readiness in other parts of the globe.
As the conflict grinds on, President Donald Trump's talk of finding a quick end to the war also appears to be fading.
He stressed last week that he's "not in a hurry" to get Iran back to the negotiating table, and he continues making the case that the Islamic Republic's leadership is on the ropes.
Appointments to Iran's senior security leadership in the past month, however, have signalled Tehran's defiance after weathering decades of sanctions as well as a lack of tolerance for dissent at home.
Trump has consistently emphasised that the US and Israel campaign has been devastating for Iran's navy and air force.
Iranian officials have said the country has suffered $US270 billion ($A377 billion) in direct and indirect damage. Israeli military strikes in the first weeks of the war wiped out much of the theocratic government's leadership structure.
But Iran has found leverage through its strikes against vessels on the Strait of Hormuz, where relatively few ships carrying oil and liquefied natural gas are risking passage.
Iran still has enough drones and missiles to fire at vessels transiting the waterway through which 20 per cent of the world's oil normally flows.
Tehran continues to assert that even after it works out a management plan for the strait with Oman, its neighbour on the other side of the waterway, the US can't use it until it fulfills its obligations under the memorandum of understanding the countries signed in mid-June.
Trump has repeatedly declared that the "Strait of Hormuz is open," saying 24 vessels passed through last week. But that's a fraction of the roughly 130 vessels that transited daily before the war began.
with Reuters
