Two oil tankers, each carrying about two million barrels of crude oil, caught fire after striking naval mines in the Strait of Hormuz, Iran's Islamic Revolutionary Guard Corps (IRGC) said.
The first vessel struck a mine that triggered an explosion and a "fierce fire", while the second was engulfed "in a massive blaze", the IRGC said.
Iran's state news agency IRNA reported that both vessels had "switched off their positioning and radar systems and tried to use a route not authorised by Iran".
The IRGC said one tanker attempted to navigate the waterway "with US support", while the second was left immobilised. The names of the vessels and the origins of their crude cargo were not immediately available, says Al Jazeera.
Iran tightens control over Strait of Hormuz
Tehran seized control of the Strait of Hormuz following the outbreak of the US-Israeli war on Iran in late February, requiring vessels transiting the waterway to obtain Iranian authorisation.
IRGC naval commander Rear Admiral Ali Azmaei warned that any ship deactivating its navigation systems would be classified as "hostile".
Iran's army previously reported that two tankers had caught fire in Tehran's "prohibited zone" in the Gulf of Oman. Maritime monitors also reported that two tankers were struck off Saudi Arabia.
Six vessels were attacked on a single day the previous Sunday, the highest daily total since the conflict began.
Iran has attributed its increased strikes to retaliation against a US naval counter-blockade on Iranian oil exports and intensified sanctions.
Oil transit falls as energy prices rise
Before the war, about one-fifth of global oil consumption passed through the Strait of Hormuz, a key route for international energy shipments.
Data from Kpler showed that daily oil transit fell from 20.8 million barrels on 8 October to 2.9 million barrels on 11 October.
The sharp decline in maritime oil traffic has pushed up global energy prices and heightened inflation worldwide.
Trump rules out nuclear strikes and ground troops
US President Donald Trump ruled out using nuclear weapons against Iran, saying in an interview, "I wouldn't use nuclear".
He also rejected deploying US ground forces, saying, "No, I don't see that, I don't see it".
Trump said he expected a negotiated deal to end the conflict following the US midterm elections on 3 November.