US Central Command said it struck the three Iranian vessels on Saturday, including one off the coast of Kharg Island, near Iran's oil export hub, after Iran's Islamic Revolutionary Guard Corps launched ballistic missiles at two US Navy ships.
"Let the message to the IRGC be clear: If you shoot at two of our ships, we will impose an even higher economic cost - taking out three of yours," said Admiral Brad Cooper, head of US Central Command, which oversees the US military in the Middle East.
The IRGC responded by threatening intensified strikes against US military vessels in the region, and said later it targeted the three tankers as well as three US-linked vessels in other areas.
"Do not be deceived by the 'terrorist' US military and refrain from any suspicious movement aimed at passing through unauthorised waterways," the IRGC Navy said in a statement carried on Iranian state TV.
"Otherwise, you will be targeted."
Tasnim, the semi-official Iranian news agency, had reported that four US missiles hit a tanker in the area of Kharg's anchorage. Tasnim quoted local sources as saying there were no casualties and the tanker's crew was being evacuated.
The exchange of attacks followed an escalation in the past week that threatens to deepen a conflict that has continued since the US and Israel launched strikes on Iran in February, disrupting global energy supplies and drawing opposition from a majority of Americans before congressional elections in November.
CENTCOM later said one of the tankers had sunk due to the "precision and professionalism" of US service members.
The M/T Kylo had "joined Iran's navy at the bottom of the sea," CENTCOM said.
The tanker was not carrying oil, according to the statement. A video clip released by the US military initially appeared to show the hit vessel listing, before it sank.
While US President Donald Trump's administration seeks to pivot toward an economic clampdown on Iran by widening sanctions, the president himself has kept open the possibility of military action.
On August 31, Trump issued a one-line social media post accompanied by an AI-generated video depicting Kharg Island being "blown to smithereens".
Iranian authorities have vowed a strong response if Kharg is attacked.
Iran is the third-largest producer in the Organization of the Petroleum Exporting Countries and exported 90 per cent of its crude via Kharg Island before the war. Flows have been disrupted since a US blockade of Iranian oil exports began in mid-April.
Trump has spoken of an attack on Kharg before. He said on June 11 that he wanted to take over the island, but then said a US military operation there was off the table, days before Tehran and Washington signed an interim agreement aimed at ending the war on June 17.
A US attack on Kharg would further pressure Iran's oil industry and its wider economy, which is already reeling from the US naval blockade.
The US imposed its blockade on Iranian ports after Iran effectively shut the Strait of Hormuz, an important waterway that carried one-fifth of the world's oil supply before the war.
Brent crude futures closed at their highest level since July 24 on Friday, at $US96.28 ($A133.85) a barrel, as tensions in the Middle East drive global supply concerns.
with dpa
